It stops most of the heat. I'd say it's about the same as carpet without padding. The trans tunnel would get too hot to touch for more than a second, and now when I press my hand there it just feels a little warm. Not bad at all.
It hasn't melted yet, and I doubt it will. But if it does I'll report it.
I don't know how heavy it is. Definatly heavier than the carpet+padding or herculiner. The whole roll probably weighed about 50 lb, and I'm probably using 3/4 of it in the passanger area. There wasn't enough to do the very back, so I'll do that later. If I get another roll of the same stuff I'll weigh it.