Just measure your doors. You will want to hit both the inner and outer surfaces so multiply by 2. THen multiply by 1.5 to allow for 2 layers where it is most needed. Should get you very close to what you need.
So you were going to trade a JL 300/2 for 12 sq ft of Stinger Road Kill????????? Umm...how bout I just kick you in the balls and you give me the amp? Sounds like a better trade to me.
All joking aside though, Stinger Road Kill sucks. You basically have to clean the surface 20 times for it to stick. You also HAVE to use a heat gun or it will fall off. It also isn't very thick or plyable. Get Dynamat, Raamatt, or either Hushmat. They are all really good.