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.
or better still a 1D4 @f/8 significantly outreaches the 7D
Try a 1D4 with 600 + 2x for starters .....
Sure, but I think you missed my point...your spending over $18,000 for that 1D IV, 600 f/4, and a 2x TC. If you need 1560mm, then your probably in a position where such cost is easily budgeted...but on the flip side, you could get 1920mm out of the same lens and TC if you used a 7D instead of a 1D IV. All things being equal, for birds/bif, I'd still take the 7D (or, preferably, a 7D II with much-improved APS-C sensor with slightly more MP, higher max ISO, and less noise overall.)