Dunno, never had a pc that was that screwed for it to be a concern:/
IMO it should be more than fast enoughm but I'm not sure if the software will work, as I doubt the developer tested it on the assumption that the mouse would be connected this way.
If not just go USB to PS2, you proabably won't reach 250, but defo more than 125.
It will work, the operating system will see the new USB-controller just as the ones integrated on your motherboard (which is connected by a PCI-bus too by the way). You will only be "overclocking" the polling rate between mouse and USB-port by using the "usbhidf" program, which will work on driverlevel (and so is able to only affect the human interface device).