Can you be more specific? Are you referring to Shadow areas? If you're happy with the 7D then the 5D (IMHO) handles noise and resolution so much better
I've got some great bird shots, all cropped off the 5D III with a 400mm L 5.6 at my link if interested
http://www.flickr.com/revup67 (won't post photos to avoid clutter)
Some very nice shots there.
I'm not sure how the 400mm prime compares to the 100-400 zoom for AF.
yes, shadow areas and lower midtones are my concern with both 5d2 and 7d, and even the 5d3 frankly, it's not that much better at low ISO than the 5d2. They're all relatively decent at higher ISO.
I'm not happy with the IQ of the 7D or any of the 5ds for landscape work so I limit their use to avoid their weaknesses.
5D's pixel density is half of the 7d's but yes, the quality of those fewer pixels is better because of the better SNR.
I've considered this trade before but, because that still leaves the 5d3 as a one-trick-pony in my stable, I can't justify moving to it just for this purpose at this time. If I didn't already have both of those bodies the 5D3 would be a clear choice.
IF the 5D3 price drops to about $2500 and stays there, I'll consider the trade only because...
.. I can't find a zoom that's comparable to the 100-400mm L that works in the Nikon lineup.
If I did, I'd have my answer!
with Canon alleged to be improving their sensor tech in the next generation of cameras I have another reason I don't want to buy a 5D3 now. 7D2 might be much improved or another FF body may appear that will do a better job. Then I'd have a better case to justify a new FF Canon as I could also use it with their excellent T&S wide angles.
As it is right now and for the foreseeable future, Nikon bodies are meeting most of my critical needs much better and will likely continue to do so for the next year or 2.