The majority of road noise will come from the floor, fender wells, and cargo area... I noticed a 2db reduction from deadening my floor . the only places IMO that foam belongs, is places where structural bracing and sheet metal can hit against each other.
Id stay away from Higher End applications like Second Skin and Dynamat, no matter what you do its gonna make a difference. I would only recommend higher end stuff if you were planning on doing some car audio work to it. But in terms rattle, even the cheap stuff will be just fine. Look into Fat Mat and Rattle Trap. Best bang for the buck in my book.
As for this, how many layers of the "cheap stuff" is needed to make the same difference one layer of higher end product ? the cost of Peel and seal times the amount of layers needed almost equals the price of the good stuff. Deadener does not stop rattles 100% , hence why expanding foam or additional bracing is needed in certain areas.