Very interesting stuff here.
I can say firsthand that the stiffness of OE rubber mounts can vary greatly, they're not all soft. My last ICE car had lots of truly soft mounts from the factory, and the automaker themselves had a whole catalog of upgrade parts. Many of the mounts were just stiffer rubber that looked IDENTICAL to the soft factory parts, save for the part number. They weren't identical though! I upgraded all the drivetrain mounts with the stiffer rubber and it made a big, obvious difference. Felt awesome, stiffened up the shifting, handling, and even made the engine sound meaner.
The stiffer rubber wasn't racecar stiff, the automaker offered even stiffer, mostly solid mounts for that. The stiffer rubber I went with was a nice compromise for a daily driver though. For street driving, even aggressive canyon carving, they felt great. On the racetrack I could certainly tell there was still movement of the drivetrain, I could feel the tranny moving especially for shifts like 2->3 (regular shift pattern, not dogleg). It was much better than with the original soft mounts though where I could barely find 3rd gear!
For that car I got rear subframe inserts analogous to these. Installed along with other mount and bushing upgrades at the same time so I can't say what effect they had individually, but the cumulative effect was great, really tightened up the car's feel and responses.
It's helpful to know that the M3P factory subframe rubber is pretty stiff already. I'm not tracking my M3P so I really don't "need" these inserts...but that didn't stop me from some other mount and bushing upgrades that I certainly didn't "need"...okay okay control arms first!