I saw a review of the Sigma that said the same thing.
For my own tuppenceworth. The sigma 150-500 is meant to be better than the Nikon 80-400 at 400.
The nikon is an old lens. And is to be hoped Nikon will upgrade it soon.
You can get flight shots using the Nikon
http://1.bp.blogspot.com/-Z6WumK87fi...0/DSC_1463.jpg
http://1.bp.blogspot.com/_oG9pguvp-2...h/dsc_7772.jpg
http://4.bp.blogspot.com/_oG9pguvp-2...h/dsc_7709.jpg
it was hand held.
Not sure about the 70-200 with a 2x convertor to get to 400m especially with the focus breathing issues. If it was me I'd go for the 300f4 with a 1.4 TC. It lacks VR but with your d7000 you can push the ISO so you can get 1/500 shutter speed. And I bought the 80-400 instead of the 300f4 so do as I say not as I do
alas Nikon are behind canon at this price point.