The way I'd formally test would be to pick a few lenses and measure the time taken to rack the focus from MFD to infinity (with the lens cap on) on various bodies.
Obviously, though, that's only raw AF motor speed. In the real world, type and quality of the AF point, the AF system's algorithms, and the subject will play a big role - probably more than the actual AF drive speed.
The IQ penalty with the 2x is real, but minimized on top end lenses like the 70-200 II and the supertele lenses. I'd still advocate getting the focal length needed, if possible (budget and other factors notwithstanding), rather than planning on constantly using a TC with a shorter lens.
i just wondered, does a batterygrip on for example a 7D, add more juice to the system? and therefore let you have your lens focus faster?
The grip has no effect on AF speed. The two batteries are used alternately, not simultaneously, so there's no voltage boost with the grip. The LP-E6 battery used by the 5DII/5DIII/7D/60D puts out 7.2 V, whereas the LP-E4 used by the 1-series bodies delivers 11.1 V.