Harder rubber will add some NVH. But why go to such lengths to change these bushings out? Are they bad? If I were dropping the subframe for some reason, sure. But I wouldn't just do it to do it. The E46 had known issues with the FCAB, RTABs, and either the diff monts or the subframe bushings (one would go, the other would be OK). I'm not aware of any issues with the E9X platform.
I also think the E9X M3 suspension design is more durable than the E46 design.
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