I had no trouble with f/8 AF on my 7D, my 40D, or even my old 10D when using a Kenko DGX TC and my 100-400mml. There is a incompatibility with a 100L, but AF works well with the Kenko. No need to worry or wait if you need it, its been here for a long time.
I also have the Kenko DGX. I purchased it for the explicit purpose of using it with the 100-400mm. It works, but only in stellar light. During the times of day I needed it most, mornings and afternoons just after and just before sunset, it pretty much did not work at all. It is really a mixed bag when you do f/8 AF on a camera body that does not officially support it. When it does work, it is very slow. Hunting occurs frequently. When I have great direct sunlight, it will usually work, but I've also noticed that it lacks precision...when it "locks", focus is usually off just a little bit. It is random, whether it focuses front or back, so the 7D AFMA is not helpful when using it. Further, CA and IQ become very problematic when using the Kenko with the 100-400.
I've used the Kenko with the EF 600mm f/4 L II, and its performance is stellar. Focus is pretty fast (although I believe the primary limiting factor here is actually the 7D, I hope a 5D III will be faster), and it is pretty accurate. Precision is good. CA definitely increases with the Kenko, and depending on the light and atmospherics, IQ can suffer to an undesirable degree.
It is a good TC, but I would not go so far as to say that you'll get flawless or even great f/8 AF performance out of it on a body that does not officially support f/8 AF. You might get some good performance at some times with the right kind of light, and at other times you may get nothing but full-range hunting and zero focus locks.