Folks, if you already have a 100-400mm or 70-200mm f/2.8L IS II, then stick with what you have. I actually had all three of these lenses and, well, kept only one. The 70-200mm f/2.8L IS II is so sharp, I don't see much difference between it and the 100-400. When using the 1.4x III or 2x III, the image quality is suburb. Stop is down one stop and it gets a bit better.
Here are some I took last Sunday using the 2x III (two at f/5.6 and ISO 3200 on a 60D, last one at f/8 ISO 640). None of these would have been possible using the 400mm f/5.6L. My shutter speed was 1/200s or longer.