No, I did some deep testing, USB virtualization doesn’t seem to be limiting performance/bandwidth in any way.
If you’re not using the latest 2.0 ZS release, try upgrading. The problem seems to be gone for me since I have switched to rev 2.0 (although I need to test it further).
A VMWare virtual machine is not yet available for 2.0, so you’ll have to build it your own (not a difficult task anyway).
So the problem might be depending to the AT command sent during initialization of the USB stick? Anyone can say how to check the link speed?