It just helps eliminate some road noise. I have the same stuff in my front doors and they sound a lot more solid when you shut them, vs. having no insulation at all.
There is plenty of room in the bottom of the door for the speaker, I used to have them installed in there before. For some reason Richard moved them to the kick panels. But, my new kick panels are much thinner and I'm afraid the weight of the magnets in the speakers might warp them, plus it will sound better to have the speakers opposing each other by mounting them in each door panel. The panels themselves are just masonite covered with vinyl, and they're flat, so it should be easy to cut. Are you coming to the cruise-in tomorrow?