I'm afraid I may not be a lot of help to you on that. I did a quick search on Google and found a forum post on Futuremark
about 6950's not being seen properly. Doubt it has any bearing on your situation though.
If it were me I'd monitor the card temps at default clock speeds, 1250, then with the clock changed to 300 (even though GPU-z shows 1250) and see if there is a significant difference. I'd venture a guess there will not be much difference between default and 1250, but at 300 it should drop a few degrees. If it does drop then there is a good chance GPU-z is not reporting the right clock speed. If it stays the same I'd believe GPU-z is reporting correctly.
Just some thoughts.