Try 11.6. Some clocking things weren't working in earlier drivers and 11.6 was the last good one.
I'm striking out..
11.4 can't OC past predetermined limits (840 on 6950)
11.6 no change (was running 11.6)
11.7 no change
11.8 no change (and slower..

)
I'm beginning to think I'm missing something basic.. off to do a fresh OS install, and try again.