As you indicated hat mirror issue made it neccesary to exchange a classic wide-angle design for a retro-focus design, probably introducing additional abberations.

Nonwithstanding any achievements in retro-focus desings on which you inquire, the retro-focus principle lets the imaging rays fall onto the film with steaper angles than with the classic concept, thus the cosines-law will have less effect. Thus no need for a correcting density filter and by that a virtual speed gain of such lenses.