Which 2x TC for my 70-200 2.8LII ?

Levina de Ruijter wrote in post #18532762
... Much to their own surprise the Kenko 1.4 MC4 DGX, the most humble extender of them all (and the cheapest), was the best overall.

They must have a different meaning for the term "overall"

With a modern Canon body, on certain lenses the MkIII T-Cons offer AF capabilities that the Kenko can not match. so maybe "overall" actually means something more specific?

Sadly the OP's 6D will not be able to take full advantage of the MkIII T-cons specific AF advantages.

TeamSpeed wrote in post #18533194
The MKIII of the Canon TCs address some of the AF speed issues, I believe....

On a 6D there will be NO difference in AF performance between the MkII and MkIII sadly.

Larry Weinman wrote in post #18537028
It is my understanding that the 70-200 f2.8 ll is the only series ll lens that does not benefit by the chip in the Canon 2X lll teleconverter. I tried the 2X lll on my 70-200 f2.8 ll and it is a dog as far as AF speed and the AF gets easily confused when focusing near the minimum focus. I have the 100-400ll and that focuses quickly even when the 1.4 is attached.

Fascinating. The 70-200mm II is one lens I have not had a chance to try the MkIII.

TeamSpeed wrote in post #18537031
I am pretty sure the AF improvements are restricted to the long fast primes like Canon EF 300mm f/2.8L IS II USM Lens, Canon EF 400mm f/2.8L IS II USM Lens, Canon EF 500mm f/4L IS II USM Lens and Canon EF 600mm f/4L IS II USM Lens.

100-400mm MkII.
When using 5D4 for example, the difference between MkII and MkIII T-cons is substantial in the AF dept. None of Canon's previously imposed mandatory AF slow down "for focus precision" and all points active @ F/8.