I installed some FatMat (wannbe DynaMat) in all 4 doors, under back seat, rear shock towers, whole trunk. WOW! What a difference! That familiar Jumbo Jet-landing-on the-runway sound is a whole lot quieter!
I can't say it enough how happy I am after doing this.
I think I only added about 10-15 lbs at the VERY most. I took out my jack and spare a while ago so it more than equaled out.
$49 on ebay for 36 sq ft. and I still have 2 sheets left. I might do the firewall next.