OK, here is what I have, all tests with EOS 5D Mark IV
In a nutshell, you get AF @ f/8 with every T-Con combination I tried equaling f/8.
If you want to work with all AF points, it has to be a MkIII T-Con
EF 500mm f/4L IS MkII With EF Extender 2X MkII RESULT: AF Works FINE! 9 AF Points are selectable.
AF points three positions left or right, and 1 position up or down from center are available. Put another way, 7 across center, 3 up and down center. Total 9 AF points.
EF 500mm f/4L IS MkII With EF Extender 2X MkIII RESULT: AF Works of course, All AF Points I think.
Lost count, looks like an awful lot of AF points selectable! 11 across, 7 up and down, and you don't need to stay in line center.
EF 100-400mm MkII with EF Extender 1.4X MkII RESULTS: AF Fine again. 9 AF Points are selectable.
Same as 2X, only 9 AF points available for selection. Same result with SIGMA and Kenko 1.4X
I don't have a Canon 1.4X MkIII yet, but suddenly I have a reason to want one