Before the comment from the digital picture gets too lost, whilst he did say he felt the tracking AF was slightly slower than the 2.8 primes he also pointed out that with the zoom he got a lot more keepers per event because of the flexibility it gave him in framing.
The worlds best sports pros were very happy to use the 200-400 at the Olympics, a four year event that can be a career builder or breaker, they wouldn't risk using them on a whim. The lens is a superb lens and anybody worrying about f2.8 vs f4 is worrying about the wrong thing.
And this I have to agree with, half way through 4 weeks in Tanzania & South Africa (overnighting JoBurg), the 200-400 is just living up to every expectation I had, every one, this is simply a Superb Lens for Wildlife, nothing better, sports etc I cant comment as I don't shoot it, maybe in Sport the Primes will work better, I don't know, but I have the 300f/2.8/400f/2.8 both with me now, and I've pretty well left them at Camp after a week of shooting.
Early morning & night time shooting with a spotlight, the f/2.8 has a role, but as soon as you have light to shoot at f/4 to f/5.6, the 200-400 just doesn't have any competition, the ability to Track a subject at 560mm f/5.6 coming towards you then flick out the converter as it approaches and smoothly maintain the subject in focus then zoom back from 400 to 200, just changes the Game for me, previously I'de be juggling the 400 then the 300 then the 70-200, so 3 Bodies set to go, now I pretty well get away with 2 1Dx Bodies hooked up to the 70-200 and the 200-400.
I haven't compared the long end of the 200-400 @ 560 yet with the 600f/4 as I was already carrying too much gear, but Svalbard in August for Polar Bears will give me a chance to see how theses two compare.
I tried the 1.4 converter added to the 200-400 with 1.4 engaged, Image Quality is acceptable, not good, but usable, with the 2x converter installed with the 1.4 engaged, forget it, you can do it, but not sure why you would, Image Quality is Crap.
Focus tracking on two Cheetah Kills in the Serengeti was superb, the only time I lost Focus was if the Cheetah went completely behind a Bush, the Focus would begin to hunt, re acquiring was never an issue.
Weight, well I may now have the strongest right wrist I've ever had, this combo, 1Dx + 200-400 is no light weight, it's heavy, but perhaps half my shots to date have been hand held, i hand hold the combo much more than I ever did with the 1Dx + 400f/2.8 V2 Combo.
Attached shot 1Dx 200-400f/4, shot @ 236mm, f/7.1 & 1/1000th, ISO640