To answer the original question:
There are several differences, the most important I think is that the Ground Control RSM is made for an e30, and the IE seems to be "universal". The GC reinforcement bracket is for an e30, but the IE looks like it is for an e36. I can only guess, but it looks like an e36 part sold for an e30.
Somebody mentioned TCK, but that is also universal, and has no reinforcement at all. Plus the bolts need access from the trunk, which nobody else requires anymore. As far as rubber vs urethane goes, any RSM is a good application for urethane. There is no twisting or rotation involved on an RSM, so the squeaking that comes from most urethane suspension bushings won't happen with any urethane RSM.
Monoball rear shock mounts are just silly for anything that is not a full blown race car, and even then you need some expensive shocks to have difference, let alone feel the difference. Monoball RSM will --always-- make tons of noise.
I have measured the deflection of a GC RSM on the dyno vs a monoball vs a rubber one, and the deflection is:
Monoball: 0.0
GC.0.007
rubber (buna!) 0.120
Ground Control for an e30
Ireland Engineering
Tc Kline
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