In trying out my new quartz dielectric diagonal this weekend with my AT6RC, not only did I have to remove both 2" extensions, but with the focuser dialed all the way in and locked down...my camera was in perfect focus! It seems to me that if the diagonal is simply a reflection plane at 90 degrees, then this must be all about the AT Ritchey-Chretien telescope design. Amazing. I've sent an email off to Astronomics to see if they or the manufacturer know if this was intentional.
I've attached a picture below if you're not sure what I'm talking about.