After reading that the 1.4X Kenko PRO 300 DGX was autofocusing at f/8 with Canon super teles on 1D X Bodies, as well as reading the experiences of some who posted here with the 5D Mark III, I decided to go for it.
My TC arrived late this afternoon, and I decided to give it a try on my 100-400mmL. At 100mm, its a equivalent of f/6.3, and the 5D MK III indicates f/6.3.
I tried it indoors in fair-good light, and it autofocused smoothly and quickly. Then I zoomed out to 400mm which is f/8 equivalent, and I was pleased when it snapped into focus repeatedly.
Then, I decided to give it a tough test, and took it outdoors where it was rapidly becoming dark. This is the area where TC's usually fall flat. I zoomed out to 400mm on a far distant house with the sky still a little bid bright, and it did a fair job of autofocusing. slower, but suprisingly fast for the situation. The ISO level was 8000, but the image under exposed.
Then, I tried it without the sky in the background on my rose bush in deep shadows, but it would just hunt. This was a very dark area. Finally, I tried it on my bird feeder at ISO 12800, which was a correct exposure, and it smapped to focus quickly. I was amazed.
Its too late to do any more testing tonight, but I'll try testing other lenses and combinations as I can make time.
Already, I can say its worth the price over a Canon 1.4X MK II TC as long as you do not need weather sealing.