I've dynomat'd a few key places (backs of outer panels) but that's not what I'm talking about. I used Thermo Tec DuraKool, which is like a felt mat kind of thing like what's under your carpet already. I just ran that under the carpet and to replace what used to be on the firewall. That was a noticeable difference.

Help/hurt performance? Seriously? If your car is so lame that 6 pounds of felt positioned 5" off the ground is going to make a noticeable difference in acceleration or handling, then you have one sad ass car. Crank that up to 40 pounds and you *might* have something. I'm not picking on anyone specifically here, just in general, sometimes people here act like there's a secret underground competition to see who can make their 240sx the most unbearable, undriveable pile of crap that will make it to work and back. It's friggen sound deadening. Geo Metro's come with it stock! There's more weight in OEM paint on my car than there is in sound deadening I've added. Then again, you're talking to the guy that welded on a 5 pound muffler to quiet down his external wastegate dump.