Some of our forum members swear by this. I have a roll, but not installed yet. Often goes on after first dynamating. Then use this after. They each satisfy a different requirement.
I recall reading strong negative using the stuff you are asking about. Forgot what, could be something about elements that a car is exposed too
Photo is of a friends car that he is currently building. Dynamat and then Aircraft Spruce. Excellent installation. Doors yet to be done.
http://www.aircraftspruce.com/catalog/appages/insulator3.php