I literally almost bought the Kenko 1.4x yesterday, to try out with the 100-400mm for those times when I can't bring the 600/4 II. Now that the Canon 1.4xIII will soon give me AF, I've just ordered the 2xIII (to replace my 2xII) for use with the 600/4 II.
Side note - the Kenko remains a viable option for those with a 70-300 L, which is physically incompatible with the Canon TCs.
My Kenko 1.4X TC is still within the return window, but I'll keep it. As noted, it works at f/8 with many lenses that do not accept a canon TC, and even focuses at f/8 with my old 10D!
I do hope they release a similar fix for the 5D MK III, but they will likely delay, if only to find out about any bugs once a large number of users have the firmware.
I decided to do a little testing of my 5D MK III and Kenko TC this morning, but could not format my CF. After trying another CF, I noticed my battery was low, so I charged it and tried it again. It formatted the card and then froze up.
I removed the TC and everything was fine. Then I tried another camera that had worked fine with the TC and it would not work with the TC either.
So my TC has died. Back it goes. I'll hold off on a replacement until we see if Canon does upgrade the 5D MK III, since I have a Canon TC already.