Good after Market Heat and Sound Deadener for firewall and Floor.

I have also been trying to make decisions about heat/sound deadeners for my current project. The floor pieces that just lay in place and get covered by carpet seem to be an easy thing, what I am concerned with is the self adhesive stuff on vertical surfaces. Has anybody taken this stuff off once applied? I keep my cars forever, so it's not like build it and send it out the door. If I need to make repairs or alterations in the future (like something that might entail welding) am I going to be able to get this stuff off to do the work???