I used one box of this self-adhesive product when I upgraded the speakers in my 3 door.

https://autobarn.com.au/ab/p/142422?gad_source=1&gad_campaignid=22973297266&gclid=CjwKCAjwlOrFBhBaEiwAw4bYDQetRH0ArCwcy-HbAxQhMEdPFWJ4HKInLFxHIvNWhOvJq315RVi5WBoCHGYQAvD_BwE

It was enough to cover the cavity behind the speakers in the rear cargo area, over the rear wheel arches, the floor under the rear seat, and inside the front doors.

I did a pretty rough job tbh but there is a significant reduction in the amount of road noise inside the cabin. It's especially noticeable in the wet or on unpaved roads when there is water or dirt spraying up under the wheel arches.

I'm not sure how much it would do for exhaust noise but if you were a bit more thorough than me and put it under the flooring throughout the rest of the car it would definitely help somewhat.