FWIW.. saw the comments on CR home page regarding 5D3 and AF:
>>If you remember the story about getting f/8 to work on the EOS-1D X with a Kenko teleconverter, Arthur Morris was unable to make the EOS 5D Mark III do the same thing with the exact same teleconverter. The fact that Canon hasn’t updated both cameras at the same time tells me there are indeed differences in the AF systems. I am reminded only the EOS-1 line has ever had f/8 autofocus, so it may not be something that gets added to the 5D Mark III anyway.<<
When I had my 2X TC II (Canon) and 1.4X TC ( both Canon II's.. now have the 1.4X II and 2X III), I was able to get somewhat decent Af stacking the TC's with my 300 F2.8 so I know the camera has some F8 AF ability. using 1.4X TC with 100-400L, however, results in nothing. Does not even try to AF.
I hope Canon does decide to bring F8 AF to the 5D3 but if I were a gambling person I would bet they won't, regardless of whether or not it is doable. That said a $3K body should be worthy of such a feature given nikon has F8 Af in the $2K realm.
But I can say I was able to stack TC's on my 5D3 with 300F2.8 IS and got good to fair AF. Going to keep fingers crossed on the F8 AF but won't hold my breath!