Compositing should have very little impact but in practice it seems that it doesn't work so well and there are big slowdowns.
And related to the FPS. Just yesterday I have tried a bit skyrim in windows 7 on a middle class notebook with the newest stable catalyst from amd.com. It definitely has problems with latency. Sometimes even if fraps displays 40+ FPS I see a very noticeable micro stutter. So it probably depends on multiple factors like the game, the drivers, the scheduler, etc.