i have an xfx 6950 2gb video card and i have the accelero xtreme plus 2 cooler on it. my 6950 has the power connectors side by side if your power connectors are double stacked then no the xtreme plus 2 cooler wont work. Also i would highly suggest using Antec formula 7 diamond thermal compound instead of the mx4 that comes already on the xtreme plus 2. The antec formula 7 is better then 90% of the thermal compounds out there including the arctic silver 5. I run the xtreme plus 2 cooler fans at 7v most the time while i game including playing battlefield 3 and dont reach past 56C after hours of playing. I rarely switch it to full fan speed. One important thing if you get the xtreme plus 2 dont use the 4mm spacers on the screws like it says on the manual or you will fry your 6950. The rest of the manual is accurate except the 4mm spacers part.
PS you will notice all the aluminum heatsinks dont get used up on the 6950 so I just put a couple more of them on random chips/areas on the 6950 that i thought would get hot since theres plenty left over. make sure you let the thermal glue dry on the heatsinks for at least 6-7 hours before you install the card into your case. The xtreme plus 2 is very long about 3 to 3 1/2 inches past the end of the 6950 card so might sure your case will allow it to fit in.
How did you connect the power? I have Xfx hd 6950 as well but i am forced to use the 4 pin molex so the fans runs full speed. The XFX hd 6950 2gb only got a 2 pin fan connecter but the accelero xtreme II plus got a 3 pin.