One of the very first upgrades i did to my P5 was bought the MSP #3 timing belt side mount...I purchased it as soon as Ed (theMan) listed MSP parts were available for dealership order, which was before the MSP was officially on sale...
It did make a difference with a manual P5, but was still very mild overall...wheel hop with the stock mounts was absolutely ridiculous, the MSP mount marginally helped...but the AWR front and rear 94's obviously made a huge difference...
I never heard of a MSP 'rear' mount...i thought it, too, was the same...but in either case, if you're even lightly modified...avoid stock mounts altogether...