Indeed, but my point with posting those, was that it kind of flies in the face of those reviews that say the 120-400 is very soft. Obviously there are bad samples that are, so I hope I don't end up with one.
I'm not sure I agree that all lenses get sharper by f/9. My 135 f/2 doesn't, and neither does my 70-200 f/4. By f/5.6 yes, but by f/8 there's a tad of softness.
Well, you are ready to move up to a large supertelephoto, and are ready to spend $3000 to $6000 or more. I am not. I've tried them. I'm comfortable where I'm at right now. I'm happy to just shoot larger targets in low light with my 135 f/2 (such as deer, raccoons, foxes, water fowl) and continue to save for a supertele. I have other monetary commitments right now.
The best you could do for low light birding, would be the 1Dx with the 400 f/2.8 ii or 600mm f/4 ii. If you need a low light zoom, the Sigma 120-300 f/2.8 2013 version is probably it. Supposedly it will be released mid March.