"Unless you are applying full sheets in multiple layers to try to reduce panel flex (for SPL competition purposes), the purpose of Dynamat (and similar type materials) is to lower the resonant frequency of the vehicle's body panels so that they do not "ring" when music is played. While these products claim to do all kinds of other stuff... in reality, all you are doing is lowering the resonant frequency of the panel below the audible frequency range, so it doesn't "vibrate" and color the sound with unwanted noise (either from road vibrations, music, etc). You can achieve much the same effect by sticking or smearing any kind of dense, heavy material on the panels. The main benefits to the dynamat type products are their convenience/ease of installation, and the ability of the the better quality products to adhere well in extreme temperatures.
It will not do much of anything to insulate the cabin from outside noise - road or otherwise. The reduction in road noise comes from the fact that the vehicle's body panels are no longer "ringing" from the vibration of driving down the road."
On a side note it would be interesting to see pico scope reading before and after as most of vibrations are felt/heard in different frequency hertz. I wonder what the cut in noise db would be?