I have Dodo Mat under the rear carpet, rear firewall (welded in) behind rear seat, under rear shelf and a square under my bonnet. It seems to fit under the carpet fine, even with the original factory fibre soundproofing on top of it.
The above has definitely made my car quiter and removed a lot of vibration from my bonnet caused by the WCXM. I will get around to fitting it under my front carpet at some point too, it's that good. Just not got any left at the moment.
I was also going to fit a square inside my front door skins, but when I took the inner trims off, the panels were smothered in the original spray wax, so left it.
I have also done the rear of 2 vans with this stuff and it makes a massive difference.
Additionally used it on a Mk3 Mondeo, where you have to cut the factory rubber soundproofing in the drivers footwell to access one of the front subframe captive nuts when they spin/break their tack welds.
Something else I also did around my rear inner wheel arch panels that go between the rear seat side panels and open into the boot area was to fill these with pvc loft lagging. You can buy a big roll of this in B&Q. This stuff is brilliant for lining van panels (on top of Dodo Mat) as it doesn't absorb any moisture. This area was also covered in spray wax on my car, so couldn't fit Dodo there. However, I think this has also helped reduce the panel drone. It works well there instead of Dodo because there's no window winder to interfere with.
See pics below for example of Dodo and PVC lagging.