I'm wondering why the MF's with relatively large photosites have ISO performance limited to ISO 800, or even 400 for some?
Shot noise is not as important a player in IQ as it is made out to be in most theory, IMO. Readout noise is far more relevant in most real-world cases, especially at high ISOs.
Perhaps the makers of MF cameras and backs assume that most users will be using only low ISOs, mostly on tripods, and don't bother to invest in amplification quality for higher ISOs. Maybe with CCDs it isn't all that easy to do. Recent Canon DSLRs seem to have the least amount of noise related to amplification, and they are CMOS, so perhaps CMOS is the key (I don't know how many MFs use CMOS).