It's not so much the foam as it is the fact that the foam gets the sound barrier away from the source which is the sheet metal in this case. If you were to put mass loaded vinyl on top of the dynamat or even bare metal, it wouldn't do much.
Here's where I bought all my stuff for my Sierra and learned about the different types of products. The biggest thing for me was the education behind the materials. I hate marketing people as they are only con artists trying to get you to buy a product. I want know how. You'll get that on this website.
http://www.sounddeadenershowdown.com/
I did like he suggests and went with 1lb/ft^2 mass loaded vinyl through the doors on my truck. It's an extended cab. I didn't put any MLV under the carpet or behind the rear seat initially. I didn't want to spend all that money at once since I just wanted to get it a little quieter without going overboard. The results were great! I've since added 0.75 lb/ft^2 MLV to the rear wall. I believe this helped even more. I went with the lighter stuff because I got it shipped in from HomeDepot for a good price and because I'm cheap and willing to accept lower quality for less cost. All in all, I'm really happy and Don's website taught me a lot.