I tried same experiment on my gtx 295.... FRAPS on crysis benchmark, 3d6, cod mw, I dont see any bigger swings in FPS or in frametimes (plotted) with 2 gpus enabled versus 1. In fact, comparing same runs on 3d6, 295gtx with both gpus enabled had smaller swings in frametimes than with just 1 enabled, but neither was very much.
http://img132.imageshack.us/img132/7...nglevsdual.jpg
I can tool around in a game quickly to rapidly change sceneries and record varying FPS, then disable sli and watch it look exactly the same or worse if FPS gets too low.
I was actually impressed at the consistency of frametimes for both sli and single gpu.