It's a great sound and vibration deadener. Not sure about heat, but I can't say I've ever had a heat problem emanating from my floors in this car. All that said, I wouldn't call these cars quiet by modern standards (and my top is usually down). Very, very smmoooooooth though!
Not sure on the extra weight, but it's not light stuff. Do you really notice a huge difference when you have a passenger or a full tank of gas? It's certainly not that heavy.
Some more pictures attached. All original.
By the way, the material is just laid in, shaped and cut with a scissors. No glue or adhesive of any kind.