>> which will mean I may need to take it out in PP, rather than add it as I often do!
Well, you need to make up your mind there -
My impression, having read reviews on the wider angle zoom options, is that wide open vignetting is a fact of life and in that regard the 20 Ai-S seems to hold up very well. And that is true of corner sharpness too, but maybe to a lesser extent. The newer lenses seem to do a better job of corner sharpness wide open, but then does that really matter?
The strength of the 20 is what has kept me ultra-wide-less for over 3 years now. I'm not looking to improve things at 20, though, but to get as wide as I can, pondering the 16-35 verses a filter-less 14-24 and etc. And to have an AF option because I find that focal length tricky to manually focus in certain situations, and yea, I'm getting a bit lazy after all these years.
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.