Yes it should help your mid-bass and cut out some of the road noise, clarifying the overall sound of the speaker.
Use 'Second Skin Damplifier Pro' though (exact same price as Dynamat Extreme), it's easier to re-apply if you make a mistake, doesn't need any heat to apply, and is just as good.
Make sure you do the outer door skin as much as you can (the one nearest the road) as well as the inner door skin (the one that your door card goes on) to maximise the effect. Make sure you cover over any holes through to the outer door skin to create an enclosure for the speaker which also helps.
Also if you can get some closed cell foam (Second Skin do some called Overkill), that apparently makes a difference if you put it on top of the Dynamat directly behind the speaker magnet, in that it helps stop any backwaves from the speaker and enhances the sound further.
I've been reading TalkAudio a lot recently!
