I agree that these are big lenses. I have two friends who have the 800/5.6, two others with the 400/2.8, and a number of people with 500/4's. I plan to budget for a new camera bag along with the lens. I do not expect to bring it to work every day - for example if I know it is going to rain - but as the location to photograph is literally outside the door I will not have to lug it very far.
That being said, one advantage of the 200-400 is I expect it to be significantly lighter. A 600/4 at the zoo will likely be overkill. Also, due to the different enclosures a zoom lens would be more useful there.
In terms of selling the 7D, I am finding with the 5D3 it isn't that useful. The 5D3 has so much more detail and far better high ISO support and better AF. Factoring in the resolution the 7D has a 1.44 crop over the 5D3. However, so far I have noticed that even at relatively low ISO such as 400, the 5D3 has a lot more detail. I plan to do some tests soon to figure out how much of an advantage the crop truly is - but I have a feeling the 7D will not be around for long.
So far it works like this.
Birds - 1st place 600/4 II. 2nd place 400/2.8 II, 3rd place 200-400/1.4x
Sports (baseball) - 1st place 400/2.8 II, 2nd place 200-400/1.4x, 3rd place 600/4 II
Zoos - 1st place 200-400/1.4x, 2nd place 400/2.8 II, 3rd place 600/4 II
Travel - 1st place 200-400/1.4x, 2nd place 400/2.8 II, 3rd place 600/4 II
If I approach this analytically, it works out as follows.
Judging each area equally with 3 pts for 1st place, etc. it comes out to
600/4 II - 6 pts
400/2.8 II - 9 pts
200-400/1.4x - 9 pts
Weighting the categories on a 1-5 scale however.
Birds - Very important. I do this often. 5
Sports - Very important, but it is only a few months of the year. 4
Zoos - Moderately important, only happens a few times a year. 2
Travel - Not so important. If I lack something I can always rent it. 1
600/4 II - 22 pts
400/2.8 II - 28 pts
200-400/1.4x - 22 pts
That would seem to answer the question, but I will have awhile to think this over while I am saving.