Ok, test #2. I found that if you just zoom all the way out to 600mm and f11 (with the Kenko 2x) and try to AF, it won't. But if you focus first at 140mm f8 and then zoom out, it will fine tune the focus at 600mm f11. You sort of have to get the focus in the neighborhood at f8 first, I guess, and then zoom and refocus. Still could be useful for bird and such. More testing tomorrow in sunlight.
Focus point was the pull chain.