FEBS, that matches my experience and so far, the only thing I've noticed between the 5DIII and 1D X in terms of AF is the faster acquisition related to the higher voltage battery in the 1D X. Even that isn't huge, but it is noticeable. The AF in low light might be a hair better, too, but that's a subjective observation, mainly with the 85 f/1.2 II.
In terms of AF tracking, I haven't noticed any difference at all, though I haven't challenged it a whole lot, yet. AFMA is definitely critical. When I first got my 300, I took it out for a spin with the lens calibrated, but extenders not. The 1.4x was a bit off, but the 2x back focused a lot. After calibration, the results are excellent, and with the Mk III extenders and Mk II big whites, the AFMA is used for AI Servo as well.