I don't know if this belongs in this forum (seems to me like it does) but here goes:

I wonder what OE motor mounts are replaceable and which are the stiffest ones?
I know for a fact that E28 535i or in other words M30 motor mounts can be used on E30 318is (M42).
So I'm wondering what mounts can be used on M50 or M52 for example...
I've looked through RealOEM but I can't work out if any are interchangeable...

Also attached below is a picture with M20 motor mounts that come with a type of enforcer cup (part 12 in the pic) similar to the ones UCC offers with their tranny mounts.
Seem to me like they should be stiffer than stock M50 mounts :dunno

You sure that cup is to stiffen the mount and isn't just a heat shield? There is a heat shield on the M52, but only for the passenger side (near the exhaust manifold):
realoem.com/bmw/diagrams/b/l/31.png (part #6)