I probably use my 5DII closer to 90% of the time. But, that ~10% is shooting birds/wildlife, and for that the 5DII is just not up to snuff. I'll be keeping my 7D even after swapping the 5DII for a 1D X (in fact, keeping it until there's a 7DII).Do you really need 7D if you have 1DX? 1DX has better FPS and AF system. You can simply use 1.4x or 2x TC for 1DX, right?
I use my 7D mainly (almost exclusively, now) with the 100-400mm. Since, unilke previous 1-series bodies, the 1D X cannot AF at f/8, a 1.4x TC with the 100-400mm is out. The 500mm f/4L IS II is next on the list after the 1D X...but at that point with a 1.4x, that's getting me 700mm vs. 640mm equivalent with the 7D @ 400mm - and I'm already cropping the 7D image. When you're shooting small birds, there's no such thing as too long a lens. If there were a m4/3 camera with good enough IQ and response rate, and a compatible 400mm lens, I'd consider it.
Couldn't agree with all that more! The value of a cropped sensor can't be forgotten when it comes to shooting distant action or small subjects in motion...and in that respect, the 7D is king!
There is a HUGE cost to getting the same kind of reach with FF, TC's and Supertelephoto lenses, to the tune of tens of thousands of dollars. If you try to get the same reach with a 1D X, 1.4x TC and a 500mm L II supertele as you get with the 7D and the 100-400mm lens, you would be $18,000 in the hole...and that only gets you 60mm more and some extra sharpness (which, granted, is nice...but oh so unbelievably costly!) The 7D+100-400 is about $2700-$3000 at current prices.