I know its possible to bug the game by disabling it, but according to this site http://gamingeoc.forumfree.it/?t=58867069 on problem 10 it sais that if disabled and i use my GPU's vsync it can improve performance.

Does this hold water?

I'm no expert but as far as I know this depends on how the VSync is implemented in the game. If it simply calls the graphics card driver's VSync function then there should be no difference between the two since it is in fact the same. If the VSync is realised in the engines renderer itself though it can have a performance impact if it pauses the gamecode from running until the next frame needs to be rendered. But even this is only one possible way of implementation.

Just try and see. I really doubt it though.

I use v-sync with my drivers. It overrules iPresentInterval=1 so this setting makes no difference while using driver based v-sync. The benefit for me using this is that it removes the stair case input lag while turning the camera. But this is dependant on hardware of course!

Im using a gtx 460...