Bob Atkins , Sep 04, 2012; 06:24 p.m.
It's odd. It does it only with the Kenko TC and that in itself is not surprising. I get excellent AF with a Tamron 1.4X TC and a Canon EF 500/4.5L (effective f6.3), whereas AF shuts off with the Canon 1.4x TC (5D and 7D).
What's odd is the difference between the 5D MkIII and the 1D X since they both use the same AF sensors. The 5D MkIII seems to be recognizing the Kenko as a TC, while the 1D-X doesn't seem to know it's there. My 7D doesn't now the Tamron 1.4x is there when I use it with my EF 500/4.5L
Could be a bug in Canon's firmware for the 1D-X. That's more likely than them deliberately enabling AF with a 3rd party TC. Maybe they'll fix the "bug" in the next firmware revision!