I think the question is: have you done speed tests with USB passthrough on 5.1? Some people are indicating that the speed issues are resolved without using PCI passthrough.
If you still have speed issues, have you opened a ticket with VMWare? This thread was originally about an issue with a PSOD when activating any PCI passthrough device. That issue seems to be resolved, so I doubt that they will be returning to this thread. If you are still having issues, I would recommend calling their support or posting a new thread about your issue.