Having tested two 7Ds extensively in the field (the first one was returned the second one now has firmware 1.0.9 loaded) IMO it is a good camera and overall better than XXDs, as Jia pointed out it eliminates the need for a TC given its high pixel density, in most cases a naked 400mm is all you need and more than enough. It is also light has a nice LCD and many new features not found on the MKIII which is old technology by today's standard.
I would argue that even for ISOs up to 1600 (
in good light) 7D will win due to its reach, remember that MKIII will only have 6.6 Mpixels when cropped down to 1.6X FOV. However I have to give you my honest opinion about AF and that it is not up in par with professional cameras, in many cases and in my experience it cannot grab BIF
at peak of the action moment, I have missed quite a few opportunities in the field with this camera that I would have not with my other gear. This is my opinion and it might be that my shooting conditions are more demanding, I do not photograph flying sea gulls, crows or ducks against sky or water background, most of my experience is with raptors and I am interested in flight and predatory shots. I would suggest that you rent a 7D for a few days and try it out in your shooting conditions and see if it works for you, if you find its AF adequate for your needs, then it is a clear choice over the gamble of a used MKIII.
Here is one example from 7D and 400mm at ISO 800 from this morning, when in focus, it produces excellent photos.
http://www.stanford.edu/~ahazeghi/Ph...20bpn%207D.jpg
EOS 7D + 400 f/5.6L f/7.1 1/2000 sec ISO 800
Note that this image has absolutely no noise reduction applied to it.