Tom

You're going to have trouble discussing this with folks who have solved the problem before you !

It is difficult to integrate the published specifications (which try to predict) with experience (which tries to describe). An Apo Rodagon N manages to make the transition from 15x to 20x far better than do older lenses. A perfectly flat field across a 16x20 or cropped 16x20 is hard to do. Thirty years ago a friend showed me a quick way to get the best corners I could get from a given lens in a big enlargement: lay a mirror in the corner of the image, and stop down the lens until I saw a clear, round aperture.

I'd look at the best lens you have for 16x, and see if you have stopped down far enough to get past mechanical vignetting. If not,
you simply have to try a new lens. I use a Apo Rodagon N, which I got to print a show of 20x enlargements 20 years ago. It was better at 20 x than my Apo Rodagon, which was better at 15x than was my Focotar. I think it is safe to say that you can improve over the good old lenses you've mentioned in your post.

Sorry to say I have no experience with the Apo 40 or 45. Their MTF (at 10 x !) shows the need to stop down to f/8. I have no evidence to show they would match a Rodagon N at 20x, I doubt they would. All you can do is to get one in, try it, and send it back for a 50 if it doesn't.

Just another note. You're doing pretty difficult work here, and you'll need to do all the old tricks to tune your enlarger. Don't forget to align the corners of your easel. And if you are looking to go larger than 20x, look at a Rodagon G or perhaps intenegatives and Apo El Nikkors.