Nah there is very little difference at any speed, most modern SLRs have very good damping. Certainly the mamiyas do, I can't comment on the bronnies. The biggest issue is not mirror slap, it is finger impulse, which can be easily overcome by setting the timer or using a cable release
User technique is far more important than the mirror slap, that is for sure. If mirror slap is truly a concern then just use MLU or use a leaf shutter lens (there are a few LS lenses available for the mamiya 645 systems too). ... big deal. Non-issue though.
And by the way I am an avid RF user.