There has been various debate about expectations of high ISO performance of DBs
My poorly received argumant is that the ISO setting doesnt really affect the raws file at all[a href=\"index.php?act=findpost&pid=156597\"][{POST_SNAPBACK}][/a]
That's quite possible. Most sensors are read out with only one gain, and various ISOs are obtained simply by using another amp before the ADC. If that second amp is clean, then there will only be a small benefit of using higher ISOs; the fact that the ADC-introduced noise will be lower, relative to the other noise and signal. If the amp is bad enough, then it might introduce extra noise, relative to the same absolute signal, at higher ISOs, in which case under-exposure from the base ISO is better than using high ISOs, as far as the RAW data is concerned.
It wouldn't surprise me if some of the backs that give 16-bit RAW output do so in such a way that ISO settings only change the metering, and the brightness of thumnails, while using the same gain for all ISOs.
The only way to tell is to take the proper test shots and measure the RAW sensitivity and noise levels at various ISOs. I don't own any MF backs, but I do this for all my compacts and DSLRs, and I can make more intelligent ISO/exposure decisions, based on the camera.