Glad to see this tested finally
I've been using the 80-400 for a couple of months now and am very happy with it. I've favoured it in practice over the 70-200/2.8 VR II when light allows (I haven't tried the 70-200/4). Not always sure why, though at least sometimes it's because of the extra reach and narrower depth of field (200/2.8 vs 400/5.6).
Good to see it compared against Sigmas, since they have quite a few big zooms at value price points. It'd be interesting to see a comparison against the 50-500 and 150-500 too, as they're popular alternatives to the 80-400.
I caution that the 300/4 isn't an equivalent alternative, though. In my experience the 300/4 is a pretty sharp lens - certainly better than your low-end zooms like the 55-300, but it's not as sharp as the 80-400 and has a lot more distortion (plus the lack of really useful features like instant manual focus override and the ability to zoom). Fingers crossed it gets a refresh soon, though - could be amazing.
It seems evident, however, that your chromatic aberration test is only evaluating lateral chromatic aberration. The 80-400 is indeed awesome in this regard. But it does have a sadly large amount of axial chromatic aberration. I don't know if it's better or worse than similar super-zooms or high-telephoto primes - which is why it'd be great if you'd consider testing for that in future. :)