The 80-400 is decent, I rented it once. It's not spectacular, but it is well suited to something like this. Don't underestimate the weight, it's heavy! I don't know who came up with 800mm, but is that on a DX sensor or FX? If FX, then 400mm on a DX sensor should be acceptable.

Keep in mind that you start to get haze when you shoot long distances (ie when you count in miles) so even though you've got a super long lens you're going to start losing quality (specifically, contrast)... and don't go thinking that a UV filter is going to help. (See the section about 4/5 of the way down the page about have/UV filters.)