Thanks for your interesting post on the mechanics of PST modification.
If only it were that easy! Then we would all be able to match your exquisite results with the same "kit of parts."
I see you use an aperture stop on your 6" f/8 to provide a true 120mm @ f/10.
My [rather limited] understanding is that a faster [f/8] telescope is self-limiting to f/10 by means of geometry alone.
This being due to the f/10 correction optics of the PST etalon lens group.
They won't allow an angle of incidence larger than an f/10 light cone to pass.
Do you consider is there is any particular advantage to stopping down the objective?
Reduced internal heating perhaps? Stray light reduction?
An internal D-ERF can/should only be placed where the F/8 light cone falls within its clear aperture.
Too near the objective and the light cone will heat the D-ERF supporting baffle and cause vignetting due to annular overlap.
Perhaps the 120mm aperture stop will allow a more forward placing of the D-ERF?
Less intensely focused heat, nearer the objective, due to the larger illuminated area?