The vaulted issue is out of my league, but I'd assume imaging would be great as the reflections will be naturally delayed and not hitting a parallel surface.

But the 20x20 issue can be fixed via a false wall at one end. At the construction phase it is easy and makes for a very nice and tidy set up.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire