Idk if its the driver or what, but even though I've had to lower my OC, my GPU has been finishing WUs faster than before. I know pushing your OC too high can cause problems too, but I did math throughout the OC process making sure my production went up with every mhz.
I know the TPF thing always fluctuates but if you calculate the time with the log files, you get a much better representation of your machine, and its much more stable, within a few seconds each %, not a few mins.
My GPU used to get a calculated 11:50 TPF (client varied from 9 to 14 mins!) but now I'm getting 10:58!!!! I'm pretty sure they are the same size WUs and I KNOW they are the same core (core17)
Nearly a min saved per % is about 1.5 hours per WU faster!