I put a Kenko 1.4x teleplys Pro on the 70-300L and the 5D3 works pretty well AF at F8. No need for new firmware for this combo.
You say that with the Kenko 1.4x the 5D3 autofocuses ... will this combo work with the 100-400 also as it will also result in a max aperture of f/8?
1DX, 5D3, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L II, 50L, 85L II, 135L, 17TSE, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 1.4x III, 2x III, 600RT x 4
When it comes to gear, it is better to vote with your wallet. Bitching on online forums like a lunatic doesn't get you anywhere!