According to Novak's best guess, the problem was that USB2 was limited to 1000 packets per second regardless of size. Not sure if USB3 has the same problem.
Well that's interesting, and does make some sense because USB uses what they call a "fixed frame structure" meaning each packet of information is predetermined in size, regardless of the information being transmitted. Unless there is a way to actually combine packets to make use of any unused space, it would not be possible to go over this limit.
Regarding USB 3.0 I don't really know if the frame structure was changed, however I do know that USB 3.0 is Full duplex whereas USB 2.0 is not. So theoretically this means on a USB 3.0 hub, work could be provided to the miners at the same time work is being submitted to the pool. On a USB 2.0 hub information has to take turns because its only half duplex. I would imagine this should improve the performance...but how much? I have no idea.