Forcing v=synch off causes a lot of issues, that's why they didn't put it in the menu. For one thing the physics can react VERY badly to higher than 60 framerates, which means indoors can get crazy. Also the mouse movement, especially vertical, can go nuts at high framerates.
Best to leave it on, I think.
As for my performance I have a single 480 and an older quad processor, the Q9550, stock speeds for both, 4GB RAM. On ultra settings with some shadow tweaks in the ini I get a locked 60fps indoors and usually around 40fps outside. In the heart of the big cities though my framerate plummets, which is really annoying. I don't want to lower everything in the game just for cities though, it's playable.