Dynamat is a great addition no matter which engine you have. I did all of my VS interior with Dynamat after stripping the carpets out, slathering truck bed liner then Dynamat then a new carpet set from Kirk. Careful with the Dynamat backing---it will slice up your fingers---it['s like a thick aluminum foil and is pretty sharp. Not real deep cuts but still no fun. I'd use some yard gloves if I did it again. My doors shut with a very pleasing "thunk" and the road noise is cut down dramatically. If I were ordering a new car I'd spec the truck bed liner plus Dynamat along with along list of "must haves" that we all add later.
FYI, Gordon recommends another product which is a lot less monethan Dynamat and he says it also works well. I spending money on my Speedster is a pleasure though. Part of the madness, I'm sure.