"PS I have the RZ67, AFD 645 and 645m and various lenses for them, if this helps a little."
Congratulations, you own some of the best medium format systems available! ;-)
To be honest I don´t think there will be any difference, even if you compare them with Carl Zeiss, Mamiya is a great manufacturer. If you want to see some graphs, visit the Zeiss website, they have MTF diagrams for all of their lenses, but comparing MTF diagrams from different manufacturers isn´t always practical. I always wanted a Mamiya RB67 when I was younger, but it wasn´t affordable for me back then.
Greetz, Benjamin