Supertele choices

[Dr Croubie]

With the same discliamers as JR (I would like to, but will probably never, own one of those beasties), i'd be voting Option 1.

400/2.8 II with TCs is tempting, but you're going to get the better results at the 600/800 lengths with the 600L instead of the 400L.

400/2.8 II is very tempting for using as a 400mm though. It will beat the pants off the 100-400L and 70-300L, and is a bit cheaper and lighter than the 600mm.

1Div is also an nice option, but seeing as you just got the 1DX, to me it'd be no contest. The 1Div is a very nice upgrade from the 7D (i'd be buying one if I could), but compared to the 1DX it's a lot of cash for only a few minor improvements (f/8, 1.3x) and (imho) more downsides (worse IQ, worse high-iso, debatably worse AF, less fps).
And if 7D is your last 'normal' sized body, don't forget that there's times when a 1DX/iv is just going to be too much to lug, and a G1X won't cut it, 7D would be the nice middle-ground there (unless you still have the 550D or another body?)

So if you primarily want birds I'd be going the 600L II, for closer stuff you can keep the 100-400 (maybe with 1.4xTC for wider than 600mm?).
Only get the 400L II if you really would rather take 'shorter' shots, like field sports (and then you can take the 70-300L on trips instead of 100-400L).

[HDNitehawk]

So if you primarily want birds I'd be going the 600L II, for closer stuff you can keep the 100-400 (maybe with 1.4xTC for wider than 600mm?).

You know Dr, I always heard that the 100-400mm push pull design is a dust magnet. I would bet if he goes for the 600mm that 100-400mm really will be a dust magnet, because it will be sitting on the shelf all the time collecting dust.