I have not used them but I agree that anything applied to the inside of the metal cases will reduce heat transmission through it and will probably raise the inside temperature a bit. There are a few other tricks. There are fan gaskets and special hard drive screw sets which might reduce some noise and sympathetic vibrations. Some fans are just noisy so you might want to look at quieter ones as long as the CFM air exchange is not comprimized. You want to tie wiring properly so as not to lay against fans and drives as well. I wouldn't go with sound matting however.
Has anyone tried to dampen your desktop case with sound damping things such as dynamat used to dampen sound in cars. I have my car covered and it significantly reduces the road noise, I'm thinking it would do the same thing for my pc. I'm just worried that it may increase the heat inside my case or create electrostatic or something. Thanks.