I did some comparison shots with the 80-400 and 70-200, but I used the 70-200 on the d300, while the d7k had the 80-400. The shots were done at 20 yards on a cloudy, lower light level day, so I had to up the ISO to keep SS up.
The 400mm shots with the 80-400 were sharper, better detailed than the 70-200 with 2x TC, especially when stopped down 1 stop. Some of that was due to the better noise performance of the d7k. So, I've been waiting for a sunny day to shoot both lenses on the d7k. I'm not sure that it will make much difference. The 2x TC III seems like it's worse than the 80-400, at least on my 70-200. I do think that I have an excellent copy of the 80-400. But, I'll do another test to confirm that, if the sun ever shows back up....
-L