Hey Ofer that thread was about the 2X TC not the 1.4. I don't think anyone expected 2X to be usable for flight. The new AF system in 5D3/1DX and future cameras does not even support AF with 2X on f/4 lenses to begin with...it's dead in the water.
You need to compare the 600 MKII plus 1.4X III with the 800. hopefully I will test the 600 MKII with 1.4X III soon, I don't expect it to be much faster than on MKI telephotos, but honestly I can't complain about AF speed as is when I can grab focus on fast birds like this:
http://www.birdphotographers.net/for...over-in-flight
According to Canon AF accuracy has improved though given the micro controller in series III extenders being able to talk to the CPU in the MKII super telephotos. Also the 600 MKII + 1.4X TC III will give you a 840mm f/5.6 lens that is still significantly lighter than the 800mm f/5.6, has a closer MFD and has a closer long range limiter...
Stay tuned for a field report