With regard to field sports and wanting the 300 2.8 II, would that be on a full frame or your 7D? Do you expect a 400 to be too long on a full frame to be used in that capacity?
On FF. Since getting the 1D X, the only times the 7D has been used are when I loaned it to someone as a backup camera for a wedding shoot, and to take a few comparison shots vs. the 1D X. Those comparison shots confirmed my belief (based on a more detailed 5DII vs. 7D comparison), that a 1D X image cropped to the FoV of the 7D has equivalent IQ (and better IQ at high ISO), so I'd only be giving up megapixels, and 7-8 MP meets my usual needs (up to 24x36" prints). So, at this point, I see no need for my 7D.