I had the 100-400 and don't miss it at all. IQ was fine for film but not up to digital. I'm a bit of a peep though.
Some long term use issues with the lens - dust inside, push pull tension/manual focus rings bind together. These are well doc'd online.
I have the 70-200 2.8 ii and the 2x iii and I hardly use the combo. As you know the 70-200 is a beefy lens. Now put that weight on 2.5 inch lever arm. Not well balanced on a crop but may not be as bad the 5Diii. If you go out in cold weather, wearing gloves, I've found it very difficult to keep the zoom where you want it. Since it is so front heavy your gloved hand is always on the zoom ring and it moves pretty easily.
If you have not considered the 400 5.6 I would suggest you do. It is old and has no IS. It is more $ than the 2x iii but less than the 100-400. It is sharp, easily sharper than the 100-400 and I think sharper than the 70-200 & 2x (others prefer the 70-200 2x). It is light. 5.6 is not bright and is a limit at dusk and dawn.
I really wanted the 70-200/2x to replace the 400 but it just has not happened.
550D w/ 400 and 5Diii w/70-200 and your pretty well set. (some wildlife will be a lot closer than you think)