I game off a 1.5tb USB2 drive, and have not noticed any major issues, i think 12gb ram helps, after the initial load time of an area in a game like Skyrim, the next pass through that area is pretty much instant, so i dont see USB3 making any difference other than for that initial load time.
Still, doesnt stop me looking at the drobo mini, thunderbolt attached, 120GB msata and 4xhigh speed 120ish GB SSD drives as my boot drive, yes please.