2xII vs 2xIII with 400 2.8 IS

I know the newer model is supposed to provide better quality and possibly AF speed. I've seen some pictures taken with both and they appear to be pretty close in IQ. Given that the 400 is spectacular to begin with, I doubt there is tremendous loss of IQ.

Anyone used both on a 300/400 2.8 that would be willing to say one is definately better than the other?

Not using the 300/400 but on a 70-200 myself.

I tried the mark II and the III before buying, and the II felt too soft for my taste.
The mark III beside the IQ also has better handling on CA and barrel distortions from the added glass.

Well I use it with my 70-200 II and have used it with the 135L and I will tell you it performs extremely well (2x III). I am sure it does incredible with the super tele's if it does that good with a zoom and a budget prime lens.

AF is slowed down for sure, but not nearly as bad as people make it out to be. Only when you go from MFD to infinity or so do you notice. Otherwise it is nearly instant IMO when focus distances are close. Hit rate does trip a bit, but not that significant.

Overall it is a great addition to the bag when you want more versatility without requiring other lenses.