HUGE! Sound deadning in this car makes a big difference. It cuts at least half of the road noise. I have a Stillen Exaust and HFC's and droned really bad before I put in sound deading. It was almost so bad I pulled the HFC's. After the Rammat was installed I cant hear any drone at all. It sounds like stock inside. Even driving at 80mph road noise is nominal. The foam is just a another barrier that makes it even more effective. Its pretty cheap so it doesent make sense not to use it.
I use Rammamt. you can buy it and the close cell foam here and its all very reasonable"
http://www.raamaudio.com/
Dont bother with Dynamat, its way too expensive and no more effective than Rammat, plus Rammat is easier to apply.
Its definitly worth doing even if you dont put in a sound system.
I am sorry but I was asked to not give out the measurements. All I can tell you is its exactly .8 cu ft. I will give you more detailed photos and I am sure you can replicate it.