I could be wrong, not as experienced as those posting before me, but I doubt changing the rear mount will make much difference (ie won't help reduce the vibrations), not at idle at least.

The buzz that you feel while accelerating, engine braking, or just maintaining speed might improve some, might, but if anything I'd expect replacing the rear mount w/ a polyurethane one would make vibrations(felt in the cabin) at idle worse.

I've got a new (oem style) front torque mount on the passenger side of my '92, the rest of the mounts are original. Just putting poly inserts on the driver-side front torque mount led to a huge increase in vibration, both at idle and while driving - only positive being the eerie lack of vibration at start-up
I now run 1 insert on that front torque mount, not both, which keeps the broken oem mount in the proper position but doesn't increase vibration much.

If replacing that rear mount isn't too huge an ordeal for you, I'd at least try it with the poly one installed, since you've done the others. The improvement it makes will probably be worth it if you race.
Otherwise, if you're having 2nd thoughts about poly mounts, I'm not so sure what to tell ya - I don't even want to bother w/ trying my poly inserts on the rear mount, seems like more of a pain than it's worth :shrug: