At our car audio shop we use mainly Hushmat. The major advantage with their product is that it is shipped flat packed in easy to use sizes. I have found products that are shipped tri-folded leave more waste from the 2 bands of wrinkles down the middle being harder to work with. Hushmat also requires less surface prep than other products we have used. In most of our jobs we do a layer of mat material followed by a layer of foam on vertical panels. On floors we will sometimes double up problem areas like transmission tunnels, areas over catalytic converters and mufflers for further heat barrier.
For high end jobs we go to Cascade Audio Engineering products. They have a VB-3 floor barrier is two layers of 1/8" neoprene foam that sandwich a lead core. Their VB-TSM is great for problem heat areas. It is made up of a couple layers of foam isolated by a thick vinyl barrier, topped with a aluminized mylar.
When it comes to car audio & electronics we say "More isn't better, better is better." However, with sound deadening and thermal management, as long as you are using good products the more you have the better you will be. While you are trying to keep out sound and heat is is also to remember the things in your vehicle that make their own sounds. We have all heard the ring of our door/cabin when our significant other slams the door shut, or when a sliding door is opened or closed. How loud it is when the rain is beating on your roof? Adding more material to your panels changes the resonant frequency to help eliminate the "ring" of the vehicles interior while it helps to keep outside noises to a minimum. The noise of the interior space itself need to be addressed as well. Soft surfaces like a headliner or flooring soak tons of noise.
Lexus makes on of these quietest vehicles on the road. Taking apart one of their interiors you will find materials applied directly to the metal, inches of molded foam under the carpets, interior trim panels that are made of thicker plastics with a lot of soft surfaces. Lexus door panels are some of the heaviest I have ever seen. All of this works together to make a very quiet car. Since vans and trucks are not manufactured this way we need to add mass to the body and interior trim along with all the other neat trciks we have learned along the way to quiet them down.
Nobody realizes how much vibration & heat add to drivers fatigue until it is gone. Spend 6 or 7 hours straight driving in a fully deadened vehicle and you will get out of your seat feeling more awake and fresh every time.