Sealing up the rear parcel shelf as totally as possible is the number one priority.
After that, sealing up the doors, on the inner metal panels, from the hollow of the doors.
Then the tire wells in the rear and next the fronts.
Next is the kick panels.
If not concerned with audio you could do all that with alot less than a roll of mat but I would use a few yards of ensolite, weight gain could be kept in the 15-20 lb range doing it that way.
The difference in road noise would be substantial if done properly,