It might be 1 or 2 frames behind due to the GPU and CPU working in parallel - effectively you're always one frame behind because of that, but the efficiency increase is huge. It'd be a 10-17ms delay, most noticable if something's following the mouse and you drag quickly.
That's very odd, because I'm having a lag exactly like that, but only on unlimited framerate! V-synced works perfectly. It's very noticeable if I use some behavior, 8 directions, for example, and switch to unlimited framerate.
Is the problem with keyboard presses as well as the mouse? I'm not sure what I can do about it, since Windows provides all the input... maybe on Unlimited Windows is too busy to immediately process input??