Normally I wouldn't jump in on a thread when my experience is not as broad as what you're asking for, but given that some information is better than none....
What I have used: 70-200L II with a 1.4x III and 2x III and a 70-300L
The AF speed for the 70-200 + 1.4x III is fine. Slower than bare lens but I don't find it all that noticeable. It takes a big hit (very noticeable) with the 2x III. I've found my 5D II to be a more limiting AF factor than the 1.4x III. There have been multiple threads discussing converters and the overall sense of it is that the Kenko and 1.4x III are comparable but the Canon 2x III is better than its competitors. The Kenko 1.4x is nice because it doesn't portrude out like the Canon so you can use it on other lenses (also allows TC stacking) that were not designed to work with the Canon extenders but the Kenko also is not as weather resistant as a result.
The IQ difference between the 70-200+1.4x III and the 70-300L is not very significant, but the 70-300L is more compact and lighter, which makes it a good choice for travel, zoo, etc when you know you'll be outside and don't want to be carrying a lot of gear.