The bows with "P" in front (PMA, PSA vs. MA and SA) had their risers constructed using CNC machines. This makes for a very precise riser and perfect limb/riser fit. Widow says the limbs from "M" bows (pre-2001) to "M" bows might not fit perfect. But limbs from "P" bows to "P" bows will interchange nicely. Widow also says it can, if you send the "M" riser to them they can fit to any of their limbs.
I have no experience with M risers, only the PSAIII. I wouldn't expect a performance difference.