I think Suiter touches on this, but to the group:
What would the seeing have to be in order for optics better than diffraction limited be apparent? My recollection is that Suiter makes the point that it would have to be darn good, and people hand-wring far too much about their optics not being good enough. Unless you have adaptive optics corrections in your imaging train, none of this may matter. Likely the distinction between 1/5 wave and 1/10 wave is undetectable, and for sure 1/10 wave and 1/50-100 wave is not unless tested in the lab.
As I recall, when they did the test at Stellafane with a bunch of Newts figured to 1 wave, 1/2 wave, 1/4 wave, and 1/8 wave, people could see the improvement between 1/4 wave and 1/8 wave.