Ahhh very good, this is true as im only running a 2.66Ghz celeron D in the mean time while i wait for my new i7 setup. Wouldnt it be an advantage to disable it in this case so that there isnt any further loss in performance in the card having to run logic to determine the throttling?
There shouldn't be any meaningful performance difference. The PowerPlay logic is designed to switch up a state instantaneously, but has a hysteresis of a few seconds before switching down states; so if you have instantaneous low activity then it won't switch back down, but if the activity is low continuously then it will.
If your CPU is throttling you, then the best way to keep the GPU active is to dial up the image quality settings, otherwise you'll just be wasting lots of lovely GPU cycles.... :)