Not sure about widlife shooting but in my experience with the 1D4 shooting fast action outdoor sports, the 70-200 2.8L II + 2X TC outperformed the 100-400 in IQ and speed. Additionally, zooming in/out with the 70-200 + 2X TC was much easier than a push/pull zoom that added unwanted movement to the camera even with a Gimbal head.
-1 - the great plus of the 100-400mm is as a walk-around lens for wild-life and safari or air shows and not as something to be plonked onto a tripod. When you are hand holding, movement of the camera is not a big deal.
A post Merry Christmas to all the CR mod's and forum participants. Wishing you all the very best of the new year!
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye