I was considering putting Dynamat in all four doors because I wanted better sound through the system, but I was wondering if it might actually reduce road/wind noise. If so, it might be worth it then.

Any one done it?

I havent done it on my Lancer, but I restore classic cars and truck with my father and we spray the entire inner body with truck liner and other stuff before putting in any interior work inorder to reduce outside noise and to keep the audio inside. I would say that it would help, but it will be a pain taking apart the doors to do so.

Dynamat is an insulator so yes, it will kepp noise in and keep noise out.
I want to get this done in my trunk because i get rattling from my subwoofers.
It also helps absorb the sound so it doesnt rattle against the cars chassis or anything else that is metal inside the car.