make sure you're using forceware driver 71.89 because sli'd 6600GT's should be getting better than that.
Vsynce can be on or off, doesn't matter, on would probably lower performance, I generally have it off because my image never really tears bad anyway.
I'd think you'd get about 10FPS higher, I have my GT at Ultra specs and I don't think two 6600GT's even SLI'd quite equal an Ultra