Just ordered a set of the Turner Motorsport solid Aluminium rear subframe bushings. They'll be replacing M3 rear subframe bushings that are already in there.
I've done a bit of research and it appears that replacing the rear subframe bushings with solid ones doesn't affect NVH too much as long as I keep the diff bushings stock. The new M3/4 as well as the current M5 also use a solid subframe bolted directly to the chassis without any bushings.
The reason why I want to change the bushings is to reduce the risk of wheel hop on hard launches. I've looked at the DEFIV diff lockdown kit but it's quite expensive and has more risk of introducing a lot more NVH.
All of the M3 parts have finally landed in Sydney. I'm waiting for clearance through customs and quarantine and once it's all done it'll be sent to Advan to inspect and install. I hope to have it all installed within another month or two.