The heavy sound deadening stuff is only to stop large open areas from panting/resonating by essentially making it heavier. This lowers its resonant frequency so it won't vibrate in reaction to normal vehicle movement. The deadening stuff is heavy and expensive (as you've no doubt found out) so it is a bit of a waste covering every inch possible. Just a decent sized square in the middle of the big panels will make a big difference but having been there myself, I know the temptation to cover everything!
For the rear arches you can fit thinner deadening such as Silent coat on the outer arches. This requires removal of the outer arch liner and you'll probably need to remove the factory splodge of wax-oil stuff from the arch to allow the deadening to stick. Inside I used 2mm Silent coat all over... then added 6mm foam (versus the 12mm I used elsewhere). This left a nice smooth surface to carpet but also still showed the pressings on the arch which I think looks better.
For the rear arches you can fit thinner deadening such as Silent coat on the outer arches. This requires removal of the outer arch liner and you'll probably need to remove the factory splodge of wax-oil stuff from the arch to allow the deadening to stick. Inside I used 2mm Silent coat all over... then added 6mm foam (versus the 12mm I used elsewhere). This left a nice smooth surface to carpet but also still showed the pressings on the arch which I think looks better.
