I try not to be a pixel peeper and totally accept the compromise that is using a rectilinear ultra wide angle behind a dome, but I'd still like to have whatever's going to work best. Like TheTrickster, I already have the 20mm extension ring, so I don't expect a single part (as opposed to dome plus extension ring) as being that helpful to me.
I really have no interest in using the 12-40 in future since it's just not my style of shooting above or below water. My primary use would be the Pany 7-14 f/4 and maybe in the future the Oly 7-14 f/2.8 (if it ends up being better enough than the Pany) or Oly 12/2 (since I'm planning to use it above water).
Maybe I should shoot an email over to Zen to see if they can provide some more guidance. Since the dome is a special order, I won't have the luxury of exchanging or returning it once it comes in. Hopefully, this is all just unnecessary optimization and I'll be happy either way.