A quick hands-on report with the Canon 200-400 f/4 1.4 TC by Psycho_McCrazy:
- Super-Fast AF, as expected from a canon super-telephoto, even with the 5D mk2 it was mounted on (or rather the camera was mounted on it)
- To me, the AF seemed to hunt a bit when the 1.4x TC was engaged, but then it was on a 5D2 with low light of the hall and people at various distances walking by (this lens was away from the regular canon telephoto testing area - it as on ground level rather than elevated). I am sure that with a worthy body, the AF will be top notch even with the TC engaged.
Above images are by Psycho_McCrazy, the rest by Jeff Chen: