Something is wrong, my friend has a 6850 which is weaker than the 5850 and he plays great with Ultra settings at 1080p.
Do you have the latest drivers?
What resolution and framerate? Not anything decent.
For reference, I play on high with 2x5850 @1200p getting ~90fps. Ultra is completely unplayable. Unless he's playing at 720p and thinks 15fps is good.