I was looking into Dynamat last year and figured out a few important things to know about that type of stuff.
First, most people do agree that Dynamat is the best, but most people agree that alternatives do the same job for vehicles like ours.
Second, if I'm not mistaken, Dynamat's own web site showed how their research proved that 30% coverage of any given panel is what is needed to reach 100% efficiency for what the product is supposed to do. It is not a sound absorbent material, although perceptively it has the same results. What these types of materials do is dampen the vibrations that are actually in the metal panel itself, which is what makes a lot of road noise -- the noise is the result of the vibration. But that is not all of the noise you hear in a moving vehicle. But if you dampen the vibration, you eliminate that type of noise, at least.
So when I put it in my truck I used about 60% coverage on any one flat panel (think of how many there are in a Defender!) and the difference is fantastic! The best example is on the bare metal doors in my Series truck. It makes the whole experience of opening and shutting the doors very much worth it. The door actually operates better, it isn't an assault on your ears just slamming the door when sitting inside, and it does have the effect of slightly eliminating whatever road noise from vibrations there was coming from those panels.
This leads to the third point, what you want to dampen actual ambient noise is some kind of foam insulation. I bought a roll of foam rubber with a very sticky backing that I am putting here and there and everywhere that I have a cavity. I am sure someone could find better stuff, and stuff it into places as well.
Final point is that you should use a combination of the Dynamat (or like) and foam rubber type of insulation. Unless you are putting in a high-end stereo unit, covering the entire inside of the car with Dynamat is actually a waste of money.