I've been playing with the 70-200 II with 2x TC as a replacement for the 100-400L. The IQ is pretty similar. The AF speed is pretty close, but I would say that the 70-200 II with 2x TC will hunt more if it loses focus. The 70-200 II plus 2x TC is heavier and more awkward than the 100-400L to hold and shoot for long periods of time. But, since I bought the 70-200 II and 2x TC, I've only used my 100-400L for local birding trips. It is still a great lens, but that may not be enough of a niche for me to keep it. Especially if I can get a lens even better suited for local birding (cough 500 or 600 f/4 II).

I still have to look at a few photos from my trip (computer almost fixed), but I am starting to lean toward the conclusion that the 70-200 ii is so good from 70-200 mm w/o the TC and close enough with the TC that it outweighs the inconveniences when compared to the 100-400L for general purpose telephoto photography. If I know I'll only be shooting at 400mm, the 100-400L still wins.

I will say I have heard the 100-400L called the perfect air show lens. But I haven't used it for that purpose yet myself.

BTW...great photos.