I installed Dynamat throughout the interior of my Mini (every single panel covered by a trim panel - including the floor, sides and roof) when I was 18 ish, after reading all about it and listening to mates (with modified cars) who had installed it. It was very new back then (around 2002 ish) and had only just arrived from America.
I massively regretted it pretty much instantly...
1) It didn't reduce road noise, the Mini is the shape of a brick - so if you drive at anything over 60mph it makes no difference to the air smashing against your upright windscreen (which is what creates a vast majority of the noise).
2) When driving my sound system sounded no better (it was better when I was parked up... but how often do you listen to music when your stopped, unless your at a red light, in a traffic jam etc.).
3) I'm now in the process of the restoration and removing it was the worst job I've ever had to do (I actually got the apprentice to do it a few years ago, so what I really mean is... it looked like the worst job in the world).
It did reduce panel vibration though... but personally I actually like rattles and vibrations with old cars, if you want a car that doesn't rattle or vibrate, get something modern.
I would like to add that I have since plastered one of my cars (a Civic VTi I had about 6 years ago) and we also recently lined the entire works VW T5 with a closed cell foam... both worked brilliantly, but both are modern vehicles with flat panels. My experience with the Mini was that it didn't work in the real world - it worked great when your not moving, which basically meant it made no difference to the experience of actually driving the Mini. Whereas with modern cars, it does reduce road noise and improve the driving experience (if you want your car to be quieter from the inside).
If you do want to go ahead and do it we (here at Optimise), would recommend the sheets of closed cell foam (not the foil faced product)... it's what we use on the van linings and it works brilliantly. Depending on your budget either go for a general closed cell foam or if you have money to burn go for the closed cell foam Dynamat product (I'm not sure what they call it sorry).