Well, I did some testing....
It's possible to integrate USB drivers into a WinPE boot.wim image.
Though I'm not sure how effective they would be.  To boot from a USB device requires the BIOS/UEFI to support USB booting.  That means there's an IDE-translation layer built into the BIOS which makes it appear to the OS that the USB device is a pseudo-IDE device.
As a test, I integrated the current x64 DriverPack Chipset into a vanilla WinPE x64 boot.wim.
DriverPack Chipset x64 contains 221 .inf driver files.
Of those 221, DISM only integrated 154 and ignored the rest.
I haven't checked thoroughly, but it appears that all the USB .inf files were successfully integrated.
The missing ones are probably from the "Vista" folder.
So while it's possible to integrate USB3 into a boot.wim, I still maintain that it's not recommended.  Unless someone can prove that there is a distinct benefit to do so AND post benchmarks to prove the drivers are utilized for a speed boost.
Here's the relevant section of the DISM log:
** Listing all 3rd party drivers in index 1 
Deployment Image Servicing and Management tool
Version: 6.1.7600.16385
Image Version: 6.1.7600.16385
Obtaining list of 3rd party drivers from the driver store...
Driver packages listing:
-------------- | ------------------ | ----- | ---------- | ------------------------------ | ---------- | -------------
Published Name | Original File Name | Inbox | Class Name | Provider Name                  | Date       | Version      
-------------- | ------------------ | ----- | ---------- | ------------------------------ | ---------- | -------------
oem0.inf       | 2008s4el.inf       | No    | System     | Intel                          | 2/2/2009   | 9.1.0.1012   
oem1.inf       | 5000xzvp.inf       | No    | System     | Intel                          | 2/20/2008  | 8.6.0.1002   
oem10.inf      | 965g.inf           | No    | System     | Intel                          | 8/31/2007  | 8.3.0.1016   
oem100.inf     | pbo-core.inf       | No    | System     | Intel                          | 7/29/2009  | 8.8.0.1014   
oem101.inf     | pbo-usb.inf        | No    | USB        | Intel                          | 7/29/2009  | 8.8.0.1014   
oem102.inf     | heci.inf           | No    | System     | Intel                          | 6/23/2009  | 4.2.0.1008   
oem103.inf     | heci.inf           | No    | System     | Intel                          | 7/9/2007   | 2.1.22.1033  
oem104.inf     | heci.inf           | No    | System     | Intel                          | 9/18/2009  | 3.2.20.1046  
oem105.inf     | heci.inf           | No    | System     | Intel                          | 6/23/2009  | 5.2.0.1008   
oem106.inf     | heci.inf           | No    | System     | Intel                          | 9/17/2009  | 6.0.0.1179   
oem107.inf     | mesrl.inf          | No    | Ports      | Intel                          | 9/17/2009  | 6.0.0.1179   
oem108.inf     | mesrle.inf         | No    | Ports      | Intel                          | 9/17/2009  | 6.0.0.1179   
oem109.inf     | heci.inf           | No    | System     | Intel                          | 9/22/2011  | 7.1.21.1134  
oem11.inf      | 965m.inf           | No    | System     | Intel                          | 10/19/2007 | 8.3.1.1005   
oem110.inf     | mesrl.inf          | No    | Ports      | Intel                          | 9/11/2010  | 7.0.0.1109   
oem111.inf     | mesrle.inf         | No    | Ports      | Intel                          | 9/11/2010  | 7.0.0.1109   
oem112.inf     | heci.inf           | No    | System     | Intel                          | 11/9/2011  | 8.0.0.1262   
oem113.inf     | mesrl.inf          | No    | Ports      | Intel                          | 11/9/2011  | 8.0.0.1262   
oem114.inf     | nfsmb64.inf        | No    | System     | NVIDIA                         | 3/22/2010  | 4.7.9.0      
oem115.inf     | smuc64.inf         | No    | System     | NVIDIA                         | 6/29/2009  | 5.1.2600.171 
oem116.inf     | smuc64.inf         | No    | System     | NVIDIA                         | 3/22/2010  | 5.1.2600.208 
oem117.inf     | asmthub3.inf       | No    | USB        | ASMedia Technology Inc         | 2/21/2012  | 1.14.8.0     
oem118.inf     | asmtxhci.inf       | No    | USB        | ASMedia Technology Inc         | 2/21/2012  | 1.14.8.0     
oem119.inf     | etronxhci.inf      | No    | USB        | Etron Technology Inc.          | 4/23/2012  | 1.0.0.112    
oem12.inf      | cdvcore.inf        | No    | System     | Intel                          | 7/12/2010  | 9.2.2.1031   
oem120.inf     | flxhcic.inf        | No    | USB        | Fresco Logic                   | 4/12/2012  | 3.5.46.0     
oem121.inf     | flxhcih.inf        | No    | USB        | Fresco Logic                   | 4/12/2012  | 3.5.46.0     
oem122.inf     | ftdibus.inf        | No    | USB        | FTDI                           | 3/18/2011  | 2.8.14.0     
oem123.inf     | ftdiport.inf       | No    | Ports      | FTDI                           | 3/18/2011  | 2.8.14.0     
oem124.inf     | nusb3hub.inf       | No    | USB        | Renesas Electronics            | 9/13/2011  | 2.1.27.0     
oem125.inf     | nusb3xhc.inf       | No    | USB        | Renesas Electronics            | 9/13/2011  | 2.1.27.0     
oem126.inf     | cbaf.inf           | No    | USB        | Intel Corporation              | 8/28/2007  | 1.4.22.0     
oem127.inf     | hwa.inf            | No    | USB        | Intel Corporation              | 9/29/2008  | 1.1.6.0      
oem128.inf     | uwbdfu.inf         | No    | USB        | Intel Corporation              | 6/13/2008  | 1.1.6.0      
oem129.inf     | uwbusb.inf         | No    | USB        | Intel Corporation              | 6/13/2008  | 1.1.6.0      
oem13.inf      | cougahci.inf       | No    | hdc        | Intel                          | 9/10/2010  | 9.2.0.1011   
oem130.inf     | rusb3hub.inf       | No    | USB        | Renesas Electronics            | 9/15/2011  | 3.0.8.0      
oem131.inf     | rusb3xhc.inf       | No    | USB        | Renesas Electronics            | 9/15/2011  | 3.0.8.0      
oem132.inf     | ser2pl.inf         | No    | Ports      | Prolific                       | 4/29/2011  | 3.3.17.203   
oem133.inf     | tihub3.inf         | No    | USB        | Texas Instruments Incorporated | 5/2/2012   | 1.12.24.0    
oem134.inf     | tixhci.inf         | No    | USB        | Texas Instruments Incorporated | 5/2/2012   | 1.12.24.0    
oem135.inf     | amdhub30.inf       | No    | USB        | Advanced Micro Devices         | 1/3/2012   | 1.0.0.72     
oem136.inf     | amdxhc.inf         | No    | USB        | Advanced Micro Devices         | 1/3/2012   | 1.0.0.72     
oem139.inf     | usbfilter.inf      | No    | USB        | Advanced Micro Devices Inc.    | 12/14/2011 | 2.0.7.176    
oem14.inf      | cougcore.inf       | No    | System     | Intel                          | 11/20/2010 | 9.2.0.1016   
oem141.inf     | usbfilter.inf      | No    | USB        | Advanced Micro Devices Inc.    | 11/29/2010 | 1.0.18.119   
oem142.inf     | smbusamd.inf       | No    | System     | Advanced Micro Devices, Inc    | 4/15/2010  | 5.12.0.13    
oem143.inf     | sisagp.inf         | No    | System     | SiS                            | 4/23/2008  | 7.2.0.1230   
oem144.inf     | viahub3.inf        | No    | USB        | VIA                            | 11/14/2011 | 6.1.7600.1803
oem145.inf     | xhcdrv.inf         | No    | USB        | VIA                            | 11/14/2011 | 6.1.7600.1803
oem146.inf     | viaagp1.inf        | No    | System     | VIA Technologies, Inc.         | 9/23/2005  | 5.2.0.3450   
oem148.inf     | usbfilter.inf      | No    | USB        | Advanced Micro Devices Inc.    | 12/17/2010 | 1.0.20.122   
oem149.inf     | sisagp.inf         | No    | System     | Silicon Integrated Systems     | 4/14/2010  | 7.2.0.1232   
oem15.inf      | coughaud.inf       | No    | System     | Intel                          | 7/19/2011  | 9.2.0.1032   
oem150.inf     | iusb3hcs.inf       | No    | System     | Intel                          | 5/21/2012  | 1.0.5.235    
oem151.inf     | iusb3hub.inf       | No    | USB        | Intel(R) Corporation           | 5/21/2012  | 1.0.5.235    
oem152.inf     | iusb3xhc.inf       | No    | USB        | Intel(R) Corporation           | 5/21/2012  | 1.0.5.235    
oem153.inf     | viahub3.inf        | No    | USB        | VIA                            | 1/20/2012  | 6.1.7600.1902
oem154.inf     | xhcdrv.inf         | No    | USB        | VIA                            | 1/20/2012  | 6.1.7600.1902
oem16.inf      | cougheci.inf       | No    | System     | Intel                          | 4/14/2011  | 1.2.0.1030   
oem17.inf      | cougide.inf        | No    | hdc        | Intel                          | 9/10/2010  | 9.2.0.1011   
oem18.inf      | cougme.inf         | No    | System     | Intel                          | 4/14/2011  | 1.2.0.1030   
oem19.inf      | cougsmb.inf        | No    | System     | Intel                          | 9/10/2010  | 9.2.0.1011   
oem2.inf       | 5400.inf           | No    | System     | Intel                          | 1/30/2008  | 8.6.1.1001   
oem20.inf      | cougusb.inf        | No    | USB        | Intel                          | 12/21/2010 | 9.2.0.1021   
oem21.inf      | couide2.inf        | No    | hdc        | Intel                          | 9/28/2010  | 9.2.0.1014   
oem22.inf      | dh89xxcc-ahci.inf  | No    | hdc        | Intel                          | 3/10/2011  | 9.2.2.1020   
oem23.inf      | dh89xxcc-cor.inf   | No    | System     | Intel                          | 3/10/2011  | 9.2.2.1020   
oem24.inf      | dh89xxcc-id2.inf   | No    | hdc        | Intel                          | 3/10/2011  | 9.2.2.1020   
oem25.inf      | dh89xxcc-ide.inf   | No    | hdc        | Intel                          | 3/10/2011  | 9.2.2.1020   
oem26.inf      | dh89xxcc-smb.inf   | No    | System     | Intel                          | 3/10/2011  | 9.2.2.1020   
oem27.inf      | dh89xxcc-usb.inf   | No    | USB        | Intel                          | 3/10/2011  | 9.2.2.1020   
oem28.inf      | dmi_pci.inf        | No    | System     | Intel                          | 11/7/2008  | 7.0.1.1011   
oem29.inf      | e5100.inf          | No    | System     | Intel                          | 4/30/2008  | 8.6.1.1002   
oem3.inf       | 852.inf            | No    | System     | Intel                          | 8/25/2003  | 5.1.0.1006   
oem30.inf      | e7220.inf          | No    | System     | Intel                          | 1/10/2005  | 7.0.0.1011   
oem31.inf      | e7230.inf          | No    | System     | Intel                          | 2/22/2006  | 8.1.0.1002   
oem32.inf      | e7300.inf          | No    | System     | Intel                          | 1/30/2008  | 8.6.1.1001   
oem33.inf      | e7520.inf          | No    | System     | Intel                          | 3/9/2006   | 7.3.0.1013   
oem34.inf      | e8500.inf          | No    | System     | Intel                          | 9/13/2005  | 7.2.2.1001   
oem35.inf      | esb2id2.inf        | No    | hdc        | Intel                          | 1/30/2008  | 8.6.1.1001   
oem36.inf      | esb2ide.inf        | No    | hdc        | Intel                          | 6/28/2008  | 9.1.1.1003   
oem37.inf      | esb2usb.inf        | No    | USB        | Intel                          | 11/7/2008  | 7.4.1.1005   
oem38.inf      | g33q35.inf         | No    | System     | Intel                          | 1/30/2008  | 8.6.1.1001   
oem39.inf      | ibexahci.inf       | No    | hdc        | Intel                          | 6/8/2010   | 7.0.0.1013   
oem4.inf       | 855.inf            | No    | System     | Intel                          | 10/11/2002 | 4.20.1007.0  
oem40.inf      | ibexcore.inf       | No    | System     | Intel                          | 10/28/2009 | 9.1.1.1022   
oem41.inf      | ibexid2.inf        | No    | hdc        | Intel                          | 9/17/2009  | 9.1.1.1027   
oem42.inf      | ibexide.inf        | No    | hdc        | Intel                          | 6/4/2009   | 9.1.1.1013   
oem43.inf      | ibexsmb.inf        | No    | System     | Intel                          | 6/8/2010   | 1.0.0.2      
oem44.inf      | ibexusb.inf        | No    | USB        | Intel                          | 8/20/2009  | 9.1.1.1020   
oem45.inf      | ich5core.inf       | No    | System     | Intel                          | 1/10/2005  | 7.0.0.1011   
oem46.inf      | ich5id2.inf        | No    | hdc        | Intel                          | 11/15/2006 | 8.2.0.1011   
oem47.inf      | ich5ide.inf        | No    | hdc        | Intel                          | 11/15/2006 | 8.2.0.1011   
oem48.inf      | ich5usb.inf        | No    | USB        | Intel                          | 9/13/2006  | 8.2.0.1008   
oem49.inf      | ich6core.inf       | No    | System     | Intel                          | 1/10/2005  | 7.0.0.1011   
oem5.inf       | 865.inf            | No    | System     | Intel                          | 3/25/2004  | 5.1.0.1009   
oem50.inf      | ich6id2.inf        | No    | hdc        | Intel                          | 11/15/2006 | 8.2.0.1011   
oem51.inf      | ich6ide.inf        | No    | hdc        | Intel                          | 11/15/2006 | 8.2.0.1011   
oem52.inf      | ich6usb.inf        | No    | USB        | Intel                          | 9/13/2006  | 8.2.0.1008   
oem53.inf      | ich78id2.inf       | No    | hdc        | Intel                          | 8/5/2009   | 9.1.1.1016   
oem54.inf      | ich78ide.inf       | No    | hdc        | Intel                          | 8/5/2009   | 9.1.1.1016   
oem55.inf      | ich78usb.inf       | No    | USB        | Intel                          | 8/5/2009   | 9.1.1.1016   
oem56.inf      | ich7core.inf       | No    | System     | Intel                          | 8/5/2009   | 9.1.1.1016   
oem57.inf      | ich8core.inf       | No    | System     | Intel                          | 2/20/2008  | 8.6.1.1002   
oem58.inf      | ich9core.inf       | No    | System     | Intel                          | 1/21/2009  | 9.1.1.1013   
oem59.inf      | ich9usb.inf        | No    | USB        | Intel                          | 12/3/2008  | 8.3.1.1011   
oem6.inf       | 915.inf            | No    | System     | Intel                          | 3/10/2005  | 7.0.0.1019   
oem60.inf      | ichacore.inf       | No    | System     | Intel                          | 1/21/2009  | 9.1.0.1013   
oem61.inf      | ichausb.inf        | No    | USB        | Intel                          | 2/25/2008  | 9.0.0.1005   
oem62.inf      | ichxdev.inf        | No    | System     | Intel                          | 11/7/2008  | 7.1.1.1011   
oem63.inf      | intelcp2.inf       | No    | System     | Intel                          | 10/28/2009 | 9.1.1.1022   
oem64.inf      | intelcpu.inf       | No    | System     | Intel                          | 12/3/2008  | 9.1.0.1010   
oem65.inf      | intelioh.inf       | No    | System     | Intel                          | 2/8/2010   | 9.1.1.1026   
oem66.inf      | ioatdma.inf        | No    | System     | Intel                          | 8/14/2008  | 1.2.67.0     
oem67.inf      | ivybridg.inf       | No    | System     | Intel                          | 8/26/2011  | 9.3.0.1011   
oem68.inf      | jaketown.inf       | No    | System     | Intel                          | 7/19/2011  | 9.2.3.1018   
oem69.inf      | jasperfo.inf       | No    | System     | Intel                          | 11/4/2009  | 9.1.2.1004   
oem7.inf       | 915m.inf           | No    | System     | Intel                          | 5/26/2004  | 6.1.0.1008   
oem70.inf      | nehalmex.inf       | No    | System     | Intel                          | 1/11/2011  | 9.1.2.1014   
oem71.inf      | pantahci.inf       | No    | hdc        | Intel                          | 8/26/2011  | 9.3.0.1011   
oem72.inf      | pantcore.inf       | No    | System     | Intel                          | 1/11/2012  | 9.3.0.1020   
oem73.inf      | pantid2.inf        | No    | hdc        | Intel                          | 8/26/2011  | 9.3.0.1011   
oem74.inf      | pantide.inf        | No    | hdc        | Intel                          | 7/15/2011  | 9.3.0.1008   
oem75.inf      | pantsmb.inf        | No    | System     | Intel                          | 8/26/2011  | 9.3.0.1011   
oem76.inf      | pantusb.inf        | No    | USB        | Intel                          | 8/26/2011  | 9.3.0.1011   
oem77.inf      | pantusb3.inf       | No    | System     | Intel                          | 10/14/2011 | 9.3.0.1018   
oem78.inf      | patahci.inf        | No    | hdc        | Intel                          | 3/16/2011  | 9.2.3.1013   
oem79.inf      | patcore.inf        | No    | System     | Intel                          | 6/8/2011   | 9.2.3.1016   
oem8.inf       | 945.inf            | No    | System     | Intel                          | 2/6/2007   | 8.3.0.1011   
oem80.inf      | pathaud.inf        | No    | System     | Intel                          | 3/16/2011  | 9.2.3.1013   
oem81.inf      | patid2.inf         | No    | hdc        | Intel                          | 3/16/2011  | 9.2.3.1013   
oem82.inf      | patide.inf         | No    | hdc        | Intel                          | 3/16/2011  | 9.2.3.1013   
oem83.inf      | patsheci.inf       | No    | System     | Intel                          | 6/8/2011   | 1.2.3.1016   
oem84.inf      | patsmb.inf         | No    | System     | Intel                          | 3/16/2011  | 9.2.3.1013   
oem85.inf      | patusb.inf         | No    | USB        | Intel                          | 3/16/2011  | 9.2.3.1013   
oem86.inf      | pm45gm45.inf       | No    | System     | Intel                          | 6/28/2008  | 9.1.0.1003   
oem87.inf      | qd3nodrv.inf       | No    | System     | Intel                          | 1/15/2008  | 1.3.0.0      
oem88.inf      | snb2009.inf        | No    | System     | Intel                          | 3/10/2011  | 9.2.0.1026   
oem89.inf      | tcreek.inf         | No    | System     | Intel                          | 9/10/2010  | 9.1.2.1010   
oem9.inf       | 945gm.inf          | No    | System     | Intel                          | 5/18/2007  | 8.3.1.1005   
oem90.inf      | tcrkahci.inf       | No    | hdc        | Intel                          | 9/10/2010  | 9.1.2.1010   
oem91.inf      | tcrksd.inf         | No    | SDHost     | Intel                          | 11/12/2010 | 9.1.2.1011   
oem92.inf      | tcrkusb.inf        | No    | USB        | Intel                          | 10/8/2010  | 9.1.2.1012   
oem93.inf      | whed_dev.inf       | No    | System     | Intel                          | 8/7/2009   | 9.1.1.1016   
oem94.inf      | whtpi2c.inf        | No    | System     | Intel                          | 12/21/2010 | 9.2.2.1015   
oem95.inf      | whtpi2c2.inf       | No    | System     | Intel                          | 3/27/2011  | 9.2.2.1023   
oem96.inf      | whtpoint.inf       | No    | System     | Intel                          | 11/30/2010 | 9.2.2.1016   
oem97.inf      | whtptsd.inf        | No    | SDHost     | Intel                          | 10/12/2010 | 9.2.2.1009   
oem98.inf      | wptahci.inf        | No    | hdc        | Intel                          | 10/12/2010 | 9.2.2.1009   
oem99.inf      | wptusb.inf         | No    | USB        | Intel                          | 3/27/2011  | 9.2.2.1022   
The operation completed successfully.