With that many ports, it's likely you have two distinct USB controllers in the machine. If you connect one hub to each controller, they won't be fighting each other for bandwidth. When we test at the factory, we got a bunch of machines with 6 ports in the back, 4 on one controller and 2 on another (shared with the 2 up front) so we connect one hub to each bus and let 'em eat their fill.

Also, performance on Windows is gonna suck more than *nix anyway just because of how Windows handles traffic.