Ghost 8.3 DOS has EHCI support based on the BIOS, so while USB2.0 support is built in it is BIOS dependent. You can use the boot wizard to create the type of bootable media you need with that support.
IMO, there is no difference that I can tell from using the ghost32 from a PE CD than using a bootable ghost.exe from a CD/DVD or Flash Drive. It will all depend on what kind of media you are loading the GHO from.
We've deployed images using ghost from both PE CD with Ghost32 and DOS, the only benefit I've seen to the DOS version is with a bootable usb flash drive containing the ghost.exe DOS version with USB2.0 support, WAITING for the imaging software to load is drastically reduced, as most PE CDs take around 3 to 10 minutes to fully load before the software can be run. With the DOS version, the load can be almost instantaneous because very little software needs to be decompressed to a ramdisk. Our bootable flash drives load the Ghost.exe in about a minute or less.
Other than that the load times from USB 2.0 media is just as fast within DOS mode as it is within PE mode, faster from Flash Drives than from the hard disks. We were able to load a 3ish GB image from a 4GB flash drive in about 5 minutes or so. A usb 2.0 hard disk drive was loading the same image in around 7 minutes.
So, in short, the only differences is the wait time to be able TO ghost your target, the actual imaging process isn't affected much.
And the USB speed should not be affected by DOS mode since it is a hardware level specification to run at 480mbps, providing the drivers do a proper job communicating to the hardware, it should run at the same speeds within DOS mode as it would in XP/PE anyway.