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 ).
First off, thanks for the link to AlanF's test. Very interesting stuff, and seems to empirically validate what I've experimented with in a more ad-hoc fashion. That said, it should be noted that the higher spatial resolution of the 7D is only really a benefit in focal-length-limited scenarios. If you are stuck with a specific lens (say the 600 II), unable to get closer to your subject, and unable to add a teleconverter, then the 7D will always be capable of resolving more. In all honesty, that is frequently a contrived scenario, and you more often than not are at least capable of slapping on a 1.4x TC, and in the case of the 1D X (and hopefully soon enough, for the first time, the 5D III as well) adding a 2x TC. For the more skilled, getting closer to your subject is also not a problem, and most professional bird photographers have the sneaky skill to get within feet of jittery subjects, so filling the frame and getting as many pixels on subject is rarely an honest real-world problem when money and time are no object (or at least a secondary concern).
Assuming no limitations, the superiority of the 1D X and 5D III over the 7D is very clear. I have not had the luxury of trying out a 1D X (I would LOVE to, but doing so would eat a small but measurable chunk of my savings for a 600 II lens), however I've had the chance to try out a 5D III with a 500mm f/4 L from another photographer out in the field once (as well as messed with them in local shops). The 5D III AF system is truly phenomenal, as is it's high ISO performance. The thing I like about the 5D III's AF system is its speed and consistency. I like my 7D, and its AF system for the most part...however compared to the new 61pt AF system it's consistency issues are rather obvious...it does not always lock, and sometimes when using AI Servo it never really "locks"...it jitters back and forth (particularly with medium and lower contrast detail under the AF points). When AF does lock, it locks well...but the 5D III seems to nail it and nail it well just about every time. I can only imagine how much better the 1D X must be. I definitely believe the AF system is far more important than the sensor...if you can't get the subject in focus, it doesn't really matter how good the sensor is, you'll end up with irreparably soft detail.
I completely agree that there are too many situations where, with the 7D, I am unable to achieve the shutter speed I need at ISO settings of 1600 and below. During overcast days, birds in shadow, etc. I am all too often stuck at ISO 2000, 2500 (which can be semi-usable) and sometimes 3200 or more (usually UN-usable on the 7D for artistic purposes). Between the better AF system and greater ISO latitude of the 5D III, I believe it could give just as many if not more keepers. Those keepers may not be as detailed and sharp as the 7D, but if we are talking action or ISO limited scenarios, the 7D is either going to end up with blurry subjects due to subject motion, or extremely noisy images with poorer color fidelity. I would generally agree with AlanF, that in most practical situations
, the 5D III is plenty good enough and its higher IQ/lower noise will make up for the loss of crop factor. It is for these reasons that I would love to get my hands on a 5D III...the only reason I have not yet picked one up is because of the 7D II rumors. I'd at least like to see how the IQ fares on the 7D II before I spend money on a 5D III...maybe Canon will finally get their 180nm sensor tech out the door, improve DR a bit, and improve the noise characteristics of their APS-C sensors. If they do, and slap on a better AF system, I'd probably pick up a 7D II first, and a 5D III later.
Sharpness is certainly a key factor in image quality, but in my previous examples I failed to mention anything about color fidelity. At high ISO, the 7D REALLY does suffer greatly in terms of color fidelity. You lose a lot of richness, contrast (both global and micro), and even sharpness and detail at higher ISO settings. I would say ISO 1600 is really the threshold. Below it, IQ is generally fine on all levels. Above it, IQ begins to suffer, particularly on the color fidelity front. At or above ISO 3200, you start suffering on all fronts, and few images are really keepers (and even those that may be, are usually not useful outside of downscaled web uploads). I believe I'd happily take either the 5D III or 1D X over the 7D for low-light bird and wildlife photography. I do not believe they would offer the same amount of detail...subjects would be softer at any given distance and focal length...but every other aspect should be better at any ISO setting.
If I had the luxury (or rather, the financial ability) of buying both the 1D X and the EF 600mm f/4 L II, I would in a heartbeat. I've seen bird photos taken with the 1D X, and they are beyond phenomenal. The quality of noise is stellar, it's dynamic range seems to be the best Canon has to offer right now, and the AF system is to die for. At 1200mm (600/4 + 2x TC) you have the necessary reach to pack on the pixels, so the detail discrepancy between the 7D drops considerably (although, by my calculations, you would really need around a 1570mm lens to fully close the gap...6.95^2 / 4.3^2 = 2.61x per-pixel area difference; 2.61 * 600mm = 1566mm).
I am quite certain my 7D would end up becoming a paperwieght, too, if I picked up a 1D X.