I have had a 200-400 VRI lens for about 4 years. I use it mostly for birds and travel. I have found it ideal for any use you would normally expect to use it for, except birds in flight. I don't know if it is my poor technique or the lens. I used it on my D800 to shoot ospreys flying to and from their nests last April. All the still shots are everything you could ask for. But the birds in flight seems to leave the 200-400 gasping. I don't know if it is the lens, or my inability to keep the focus point on the bird as easily as I can with other lenses. I got a 300 2.8 lens and have had much better luck with that on BIFs. I have not yet tried the 200-400 on my D4 for BIFs yet. Even though the D800 and D4 have "the same" focusing system, there's no comparison in the soeed of the two cameras.
So my experience with the D800 ans 200-400 combination - it is a fantastic duo, except for birds in flight.