I dunno but I've had a ETRS for a few decades as my main camera and NEVER seen any evidence of mirror slap or vibration even when doing macro with lens wide open. I have heard this fairy tale before about the Bronica but on challenge never had anyone who claimed it is an issue actually provide proof and I've loaned mine to them to test. I know a number of professional photographers who used the SQ and ETRS as their main tools of trade and and while some lusted over the likes of a Hassey (even I do from time to time), none ever though the camera was a limiting factor in their work.

I also had an Exacta system with the same mirror slap and even with 35mm, there was no evidence of a problem b/c of the design.