Upload & Sell: On
Except for the 1D X all 1D series cameras up this point can only AF with the center AF point, I am taking my chances with and putting my bet on the 5D Mark III, which has 41 cross-type AF points. IMHO, it would be as good if not better but you will get more AF points .
Reading the 1DX manual, lenses that become f8 with a tele-converter fall into group H which uses the center cross point only for one shot but can add 4 assist points for AI focus. The 5D3 group H description (which is still pre f8) does not describe the assist points for AI focus. I hope the firmware update will add this capability as well. I haven't seen a pre-update 1DX manual to see if the assist points were added with the update.
The 400 f5.6L lens belongs to group E (along with the 100-400) which can use the center 21 cross points. The outer 40 are useable, but linear, for lenses in this group.
Edit: Hey, this was my 1000th post!