Further to what Doug stated, it's always been possible to 'trick' Canon's f/5.6 AF points into focusing with an f/8 max aperture, with pin taping or a TC that doesn't properly report. Canon did not allow the functionality because it wasn't completely reliable. Their recent firmware, possible coupled with higher density line sensors on the 1D X/5DIII (which is how the past 1-series bodies achieved f/8 AF), allowed them to enable it with confidence.For the record, utilizing stacked teleconverters or third-party telecons (like the Kenko), I have been able to reliably autofocus with great results on the 5D3 (center region AF points), fair results (center point only) on the 5D2 and 5D, and "hit-and-miss" with the 7D.
Basically, the results I got with the 5D3 were so good that I was surprised that Canon hadn't included f/8 AF on that body right from the start. Perhaps they needed to iron out some issues first, I don't know.