The extra reach is a huge benefit of the 7D. That said, there IS something to be said about using the 600/4 or 800/5.6 on a FF sensor...the thinner DOF really helps subject isolation, and the wider aperture and lighter weight of the 600 f/4 L II IS make it an extremely appealing lens, regardless of whether you have a 7D or a 1D X.
As a bird photographer myself, the one thing I really do struggle with on the 7D with cheaper lenses is blurring out my backgrounds. At f/5.6, or f/4 with 300mm or shorter lenses, getting that nice creamy background while still getting as many pixels on subject as possible can be a tricky ordeal. These days, I love it when I rent a 500mm or 600mm f/4 L II lens...the high spatial resolution of the 7D really packs on the detail like you've never seen, and the extra stop of light means better exposures and less noise on top of the extra reach (which, in FF terms, is 800mm or 960mm, respectively).
In the other (very similar) thread, in your post with the very nice finches, you stated, "I would say the lens is the most important IQ factor. The AF system and frame rate are second. The image sensor is third,
" and went on to discuss the benefit of FF at higher ISOs. All of those reasons are actually why I prefer to use the 1D X with the 600 II, rather than the 7D. The AF is not only faster, it's more consistently accurate on the 1D X than the 7D, and the faster frame rate is nice. In very bright light, the 7D does very well; but, even in 'relatively good' light, it often takes a fairly high ISO to achieve the desired shutter speeds (especially with long lenses), and of course, birds often perch in the shadows...
I'm curious - you like the 7D for the 'reach' and 'pixels on target' especially with a supertele, but have you tested a 1D X or 5DIII along side the 7D in the same scenarios? I ask because in many cases, empirical reality trumps theory. Case in point are some tests from AlanF. Some time back, he posted some real-world testing of the 7D
(and note - only
the 7D), and came to the conclusion that, "It doesn’t matter how superior the 5D III is than the 7D, the laws of optics and information theory dictate that at 9 m you can resolve the barbs of feathers on a 7 D but you see a blur with the 5D using a 600mm lens.
" That conclusion sounds quite consistent with your comments.
Subesquently, Alan got himself a 5DIII and actually
tested it along side the 7D - he concluded
, "... the 5D III is just about as good for detail as the 7D (now my back up) and has all the advantages of much better focussing and lower noise,
" and also stated
, "...in practice the higher IQ and lower noise of the 5D III more than makes up for the loss of crop factor.
Personally, I had planned on keeping my 7D even after getting the 1D X, for the (supposed) 'reach advantage' in focal length-limited situations. In practice, I came to the same conclusion as AlanF, and my 7D has been a very nice paperweight (ok, that was a little harsh...let's say, a very nice backup camera