"My sense of the 20 is that *wide open*, I'm not likely to shoot things where the corners really matter. And by f/5.6 the corners are quite good (arguably by f/4, only a matter of degree). I think it is almost good enough to shoot a fairly decent landscape wide open. I did say "almost" . It does take a full 200% vignetting adjustment in CNX2, and some additional localized work on the corners, though. But then, landscapes at f/2.8 are silly, right? Hehe.
Just to try to explain why I think a lot of that lens and why I think you are generally well covered as it is, short of expanding further into ultra-wide territory."
Exactly, and of course I recall you have a fairly good tripod or two, should you need to stop down for a landscope or two. And as you say corner sharpness and vignetting are not often an issue wide open at 20mm, and can even be a good thing.