Sound Deadener

Your stereo will thank you

If you just covered 70% of your floorboard your stereo would sound so much better. I have just recently came back to the light of real sports cars after spending the last 4 years with a Talon and a Tiburon. I started with a 71 Mustang convertible with straight pipes and side exits though. I lined the floor of all three and all I can say is wow. I did the entire Talon (they are a hell of a lot smaller than a Mustang) and you couldn't hear anything when the doors where shut. I had over 2500watts of stereo in it too. Yes that's right 2.5/kw no typo, two JL 15" subs with two 1000 watt JL amps. I won a few comps with it but when you shut the doors it sounded like the muffler of a mini van at idle!
So to answer your question, yes it will help out a lot. If exhaust is your only worry then I would start with just the trunk floor. If you do the trunk and under the rear seat you shouldn't hear anything less than WOT! Ask any of the stereo guys. Sound deadener has come along way in the last 5years.
As far as weight you are only looking at 1/2 lb per sq ft.
Cost if you shop around isn't very high for what you need either. But please remember that you almost always get what you pay for, so stick with either Dynamat or Cascade Audio Engineering. They are the two best in the business. Even with using the best you should be able to cover everything that you need to for less than $100. Here are some links that I have to help you out
http://www.dynamat.com/news_news_rick_flores.html

http://www.cascadeaudio.com/aut1.html

http://www.dynamat.com/

Currently I do not have a Mustang, my car is a 2003 Toyota Matrix XRS.

My system is made up of four 6" mid range speakers, two tweeters, and a boom tube setup. I pump out 800 watts total and was thinking of upgrading to more power, but decided on sound deadener due to its cheaper cost.

I had the floorboards, doors, and rear hatch covered in the stuff.

My car is like a friggin coffin!
At least its coffin like when I do not have the stereo on

Huge difference!

You don't have to do the entire car though. If you shoot for the floorboards and firewall, you'll easily cover easily 60-70% of external noises.

I used a company called RAAM Audio rather than Dynamat. They came in 50ft rolls. http://www.raamaudio.com and are excellent quality.

Use your best judgement. Just remember, sound deadener adds considerable weight. I tacked on about 50lbs to my car, but for me it was worth it.

P.S., yes I'm saving for a Mustang

I've never actually used it, but FatMat is a cheaper alternative to Dynamat and seems just as good. http://www.fatmat.com/