Poly trans mount
#2
My car was louder in general, not so much exhaust noise, after changing the engine and tranny mounts to polyurethane. The don't absorb vibration as well as rubber but they definatly stiffen things up so its a trade off.
#4
Any time you stiffen the connection between powertrain and chassis you are going to get more vibration/noise in the car. Between my poly motor mounts and poly trans mount, I'd say it definitely made the interior slightly noisier, but it doesn't bother much at all.