Is the base (native iso) of the older backs 25 or is it 50? If it is 50 then do I lose anything by shooting at 25?
Caution with the term "base ISO" in conjunction with such cameras/backs, which do not offer real ISO selection (only a few MFDBs do have different ISO implementations).
As the sensor knows only one ISO setting, the ISO selection affects only the metering/exposure calculation. So, the question is, when shooting for example an 18% grey card and the result is 50% brightness, then with which ISO selection does the metering yield that specific exposure. (The 18% and 50% are only examples, I don't know the ISO specs.)
As many if not all manufacturers are cheating with their ISO specification (they make the sensors appear "stronger" than they are), the best would be to shoot a serie with different exposures and compare the results to the metering suggestion.
Back to the question: if the "base ISO" is 50, then shooting @ 25 means 1 EV overexposure and that may cause clipping. However, I doubt that MFDBs offer lower ISOs than the base; there is no point. The other direction is more important: every stop higher ISO means one stop more underexposure and pushing it in post processing. It is amazing, how much pushing the raw images of some MFDBs can bear.