Originally Posted by HiLuckyB
That is odd, My XFX 6990 will do 960/1375 @1.175v, 1000/1375 @1.205v, And 1015/1475 @1.25v.
It could be a bad card, Or it could be a bad waterblock (Read back there have been 2 or 3 bad Koolance blocks where cards wouldn't even post with the block installed) Did you make sure with gpu-z that you are getting the extra voltage?
My cards tend to mimic the figures posted by Lucky, in terms of capacity. Actually my voltage & potential clocks are nearly identical: Pushing 960 @ 1.175, 994 @ 1.2, 1015 @ 1.25. 24/7 running memory @ 1485, irregardless of the core clock. I have a pair of Koolance blocks installed & have enjoyed them without hiccup. Even cranked up voltage-wise, my vregs aren't getting further than 60 Celsius & GPUs balancing out under load between 48-52. Having "tons" of experience installing Koolance blocks (having to disconnect loop for the second made the experience seem arduously longer than it actually was), I can't imagine how their installation might result in card failure. Seems they were engineered impeccably, at least in my experience.
Remember, some cards simply can't clock as high. It's luck of the draw when it comes to overclocking..that you can't soar beyond 950 doesn't imply a bad card, just a lower margin of capacity.