I can see a pretty pronounced difference between my 15" and a friend's 18" when we set up together, especially on galaxies. Planets are pretty equivalent where cool down, collimation, and tracking matter more, but under dark skies, the 18" seems to reach almost twice as deep.
I have a 15" instead of an 18" because my little house, detached garage, and steps between mean I hand carry everything to the car and can't use the wheelbarrow handles. The 15" mirror box is my max to carry. But if that wasn't a constraint, you can bet I would have an 18" instead.
So I think there is a big jump from 16" to 20" and would certainly entertain the idea if I could handle the physical aspects of it. I would still want something in the 4" to 8" range for home use, and would not have a 16" and a 20" at the same time. But for the dark sky trips, a 20" would be awesome.
Is it possible you have a very discerning sense of objects?
I do not see much of a difference bx my 16" and my buddies 18." But every additional inch is more glass than the inch before it, because that circle the inch of glass has to wrap
around is a larger circle.
so 15" = 177 sq" of glass
16" = 200 SQ"
18" = 254
20" = 314
Hey I got a great Idea, my wife spends much too much time in her large vanity mirror. I bet I could cut a 30" mirror out of that and I just need a secondary. And the wifey could get a new vanity to boot. we both win.