I was successful at enabling SLI in the control panel, but the performance is the same with or without SLI. In both cases, my FPS is 59min/59max. I tested this particularly in the Counter-Strike:Source video stress test, but also in Day of Defeat Source, HL2 DM, The Ship and Red Orchestra OsFront.
Why is my FPS being capped? I checked in the console every time and fps was "fps_max 300".
There are two trains of thought on Vsync, I personally leave it on when gaming.
The theory behind leaving it on is that the resources are wasted doing extra works where the load changes, and you get tearing. Also if you think of the bandwidth issues, there' not much point in doing alot of extra work on frames that aren't being drawn. Caches and such also get impacted too of course.
It won't make much difference in performance, but I've seen alot of cases where for example going from 100fps to 45 to 90 to 60 to 80 to 50 winds up in the framerates now never diping below 50, smoothing out that 45.
Alot of it's preference, but there's situations for both depending on the architecture and the game played. It's a rare exception (other than benchmarking) where it makes sense to have it fluctuate beyond the refresh rate.