I have been using my 5DIII and 100-400 L for quite a few airshows. I like the results in general but the reach is a bit short occasionally. I bought a 7D to try it on airshow and sold it a week later. Yes, it gave me more reach than the 5DIII but the clarity of the 7D files is no comparison to the 5DIII's files, to my eyes.
From what I read, the 5DIII's AF is better than the D800's, maybe not by much. Of course the D800's fps is slower, BUT, BUT, the 36MP sensor will give you a lot more "cropabiliy". It will be interesting to see how a D800/Nikon 80-400mm setup is like for airshows.
I think a Canon 5DIII/100-400mm vs Nikon D800/80-400mm comparison is really an AF/fsp vs longer reach (cropability) comparison.
Now, if Sony produces an e mount version of its 70-400mm lens with OSS (Sony's version of IS) that works with the new A6000 (24 MP, 11 fps), that would be an interesting combo, if the A6000's AF is capable enough. From a a few reviews, the A6000's AF tracking seems to be great.
Sorry this went of the topic...