Well after further tests, it's not my CPU that is crippling my GPUs, it's something else (I'm seeing framerate drops in Borderlands 2 too, and I'm not anywhere close to 100% CPU usage there). The framerate drops just appear to be at random places (kinda like what
@Rice N Beans said).
@Rice N Beans what did you tests conclude? Was Win 8 the problem? If so, I think you were on to something.
Edit: I think it's a DX9 problem. Games using DX11 run fine.
Edit 2: Yes, I'm gonna stop whipping myself and go back to Win 7 this weekend. I've had it up to here with Win 8 problems.