Since you mention FPS, what FPS are you getting? If I adjust my graphics settings I do surely get a difference in FPS.
Did you try the latest WHQL drivers? Until like a week ago I used 182.50 with my 8800GTX as they were the best, however the current 191.07 (I believe its that one) really improved my whole gaming experience. I must say much more enjoyable.
As you are using SLI did you check the BI forums where there is a whole topic dedicated to SLI users.