exhaust questions

3 inch exhaust pipe for the Duster can be done. Fitment may be an issue with moves in springs. The HP loss is worth a sane unrattled brain IMO
Mufflers under the car are always loud. You could move a muffler to the rear at the tail pipes exit via a bullet style muffler or a Super Trapp. This moving of the muffler also move the source of the sound away. Insulate the trunk divider, package tray, and wheel well areas from the package tray height down to the floor as low as possible.
Create a sound barrier wall.