I am also putting different speakers in the rear deck, so I started soundproofing here. Under the rear shelf, I replaced the felt with closed cell sound material 1 inch thick. The shelf is easily removed, its held on with push plugs. In the trunk, 2-inch barrier against the rear seat. Also, filled the wheel wells by the sides of the rear seat with as much as I could jam in. Then replaced the thin plastic and felt factory barrier. This is a tight fit so you need to bend it some.
The road noise from the rear is reduced, most noticeable when going over a rough or wet road. I am going to cover the rear wheel wells, floor and trunk hatch next. A section to cover the speakers will be fitted as well. After this is done I will move forward to the doors.