I've heard from elsewhere that there will be an f/8 firmware fix for the 1DX.
So if the sensor is capable of it, and it's the same sensor in the 5D3, then it's only a matter of time before Magic Lantern hacks something into it...

(that said, f/8 focussing "works" on my 7D with 70-300L + Kenkox1.4, in that AF isn't disabled, but the sensor just isn't capable of any good results, it just hunts back and forth because f/8 is just too narrow. I suppose any camera can be hacked to 'allow' f/8, but that doesn't mean it'll work if the sensor isn't good enough...)