I bought the Kenko 1.4X, and it seemed to AF fine at f/8 with the 100-400mmL on all my bodies. After putting it on my 100L, it stopped working, so I sent it back.
Later, I found that certain lenses, the 100mmL among them will lockup the 5D MK III and require you to remove/reinstall the battery to get it working with the Kenko TC and a compatible lens once again. The problem is that there is no list of which lenses work with the Kenko TC, or which new lenses may be incompatible. As far as I know, this only happens with the 5D MK III, but may also happen on the 1D X.
I had hoped to use the TC with my 100L, I prefer a little more magnification at times.