Hmmm, i just ran 3DMark11 at those clocks (except CPU @ 3.8GHz), and i got P5167. GPU score of 4792. Now while that's hardly a difference, Gaul was getting a better GPU score than me so it's only logical to think that yours is slightly low compared. Could just be a 'bad' run though.

Your heaven problem has gone over my head, have no idea what it could be. I wouldn't bother trying the 10.12 drivers, they don't even properly support the 6900 series. Guess can always try though, just to see whether it is a driver problem or not. What steps did you take to uninstall your old drivers?

And the power slider, it just sets the maximum amount of power that can be drawn. 0% i'm guessing would set the TDP at ~150w, so 20% + or - around that. So for example, while playing normal games, your GPU would be drawing within the 150w TDP so clocks would stay at 100% of whatever was set. But in cases where a certain program will draw ~110% power, it would go over the ~150w TDP and the card would throttle clocks to keep the power draw within the set TDP. So in those cases you would have to set the power slider to +10% to make sure the power draw is lower than the TDP that's been set. Perlin noise is obviously the main example, at stock clocks and 0% on the slider, it would throttle down because perlin noise would be drawing more power than you've allowed through the power slider, but add 10% and clocks can run at full speed as you've now allowed it to draw more power. Overclocking will of course draw more power. So where +10% would normally keep clocks from throttling, with the overclock it might need to be +15% to stop it from throttling. Considering +20% is the max, this makes me think that with more overclocking (with added voltage), it would be nearly impossible to stop it throttling in cases such as perlin noise but for normal benching and gaming, high overclocks should be ok.