I have 3 keyboards on my desk: a $15 Logitech K-120 membrane keyboard, a $100 mechanical Steelseries 6gv2, and a $100 mechanical Coolermaster Quickfire XT.
First off, I never expected to see a difference between keyboards when connected over PS/2, but there definitely is between these keyboards. With the Steelseries plugged in on PS/2, mouse movement is significantly more sluggish than the Coolermaster over PS/2, and more sluggish than the $15 Logitech K-120 over USB.
If you compare mouse movement for all three keyboards connected over USB, the Steelseries loses again, and the the $15 Logitech K-120 wins. It's not surprising the Coolermaster does bad here since it's a 1000hz polling rate keyboard over USB. Plugging in a 1000hz keyboard with a 1000hz mouse bogs down your system pretty bad, and gives you noticeably more choppy field of view pans in first person games. Multiple high polling rate devices on a system does not work well. It's possible that if the USB controller was operating in MSI mode, instead of line based interrupt, it might be different, but I don't think most or any integrated USB 2 controllers will do so.
Due to most motherboards having only a single transaction translator on USB hubs, mixing USB 1.1 devices (which most keyboards are), and USB 2.0 mice, should have negative consequences as well such as described by Cypress here:
I would like to find a USB 2.0 keyboard operating at 125hz to test the difference, but I can't find any that exist. Anyway, there does seem to be a large difference in mouse response between using different keyboards, which I would never have expected until I bought three and tested them all back to back.
Tests were done using two different motherboards, the Asus P8Z77-V + Gigabyte z77 UD5H, and two different mice, Kana v2 + Logitech's AM010 mice. If any keyboard company wants to send me a keyboard to test if your keyboard is indeed crap, then I'll be happy to test it, because most of them do seem to be pretty bad. Otherwise, you should probably just reverse engineer the $15 Logitech or see if there's any benefit to developing a USB 2.0 125hz keyboard for gaming.
The Steelseries 6gv2 appears to use Cypress and/or Datacomp parts. I'm not sure if it's Steelseries fault the keyboard is bad, or one of those companies.