I took apart my 2+2's interior from rear seats-back and the doors. I brownbreaded the entire back half of the car. I also did a second layer in the rear hatch area. There was a noticeable decrease in road noise and I think it will help a lot when I redo the stereo. Realize your Z will never be a quiet as a Caddy or other luxury sedan. I think when I do the floor (and underside of the hatch maybe), there will be even less noise. That's my 2 cents.