Basically, it matters on what way you use to create the Windows To Go drive. In Windows 8 Enterprise, it DEMANDS a 32 gig flash drive. This really doesn't matter on the flash drive so much. My guideline is if the flash drive is rated around 70 MBs/sec of read speed, it's decent to use. I use 70 MBs/sec as that's roughly the speed of a hard drive read speed, usually a laptop hard drive. Write speed will matter at shutdowns as that's when Windows writes saved settings onto the flash. But again, if you shutdown a lot, a USB 3.0 flash drive with fast write speeds will matter.
The other way to go on this, if you try to backdoor it, you can use the WAIK kit (I don't know about WADK kit) and deploy the install.wim of a retail version of Windows 8 using Imagex commands to do so. This way will let you use a 16 gig flash drive, although you should use Windows 8 32 bit as I know for fact that 32 bit will float happy with 16 gigs, as it really only needs about 8 to live. But this method can only work right if you use only ONE PC at a time. Meaning, you can't run over to three PCs with the To Go drive and keep the activation status.
Also, you CAN use a USB 2.0 drive. That's how I used Windows 8 To Go, a 16 gig Cruzer drive. From my experience, boot time was almost on par to a normal hard drive, visually. It took about 30 seconds to get to Start, but it needed about 10-20 seconds to finish loading. It'll continue to pull of the system files off the flash drive and load it into the RAM as well as the page file on that PC's hard drive. Overall, once it's going, you can use it just fine. Office 2010 worked smoothly, even Photoshop CS6 worked fine. Installing the software takes a LONG time though. Office took about 40 minutes, Photoshop about an hour. Shutdowns took about a minute.
Another method for this is using a virtual hard drive. I did this with Windows 7, made me a 7 To Go drive for a while. This is a little more foolproof, as what you need to do is just install Windows 8 onto a VM with a .vhd file (using Oracle's VirtualBox) and doing everything on that. Then, using EasyBCD, you can put the vhd file onto the hard drive, create a boot record and entry, and BAM! Windows 8 Pro To Go Ghetto Edition!
Keeping activation status on this method is the same I believe as the WAIK method, one PC at a time.
I almost want to say about using USB 2.0 enclosures with a SSD. Don't. I've tried this, USB 2.0 is a bottleneck in that situation, it's just limited at about 22 MBs/sec. You can actually install Windows 8 onto a USB hard drive though. At least with the WAIK method, I don't know for sure about in Enterprise.
If you need to use Windows 8 To Go on multiple PCs, and it's not an Enterprise edition, your activation status will be declared not activated. It'll shutdown every 90 minutes.