My car runs fine, except when I try to back up while turning - then the steering wheel locks up. But otherwise, it works fine. Maybe that fits some people's definition of 'works fine' - not mine.
But hey, if locking up with AFMA means the Kenko TC 'works' then you could also say the Canon 1.4x 'works' with the 70-300L. If you zoom the lens out to the 250-300mm range, the 1.4x TC can be mounted. It will AF on the 5DIII, you can AFMA the combo, and your camera won't lock up. Retracting the zoom will bump the rear lens element into the rubber ring on the TC, long as you keep the zoom extended, it 'works' just fine.
Touche, Neuro, I clearly should have said it works fine--provided AFMA is not required. As stated in my other thread, I'm not happy about not being able to AFMA the lens + Kenko TC combo (although I'm still playing with possible work-arounds). That said, I'm getting well auto-focused shots from my various lenses + Kenko TC combos, including at f/8, and they are quite sharp.