[randywebb]

Spray glue will do just fine - I do that and then use tube stuff around the edges and in big X's on the panels.
- use the 3M stuff BTW - There are 2 types, I forget which to use.

I favor the new, lightweight, factory pad.

Also, a small air gap between the pad and the firewall could help or hinder sound isolation - it depends on the frequency, and is, as they say, non-trivial.

Foams will generally do a better job of inhibiting sound and heat transmission (the dual jobs of the pad) than will a fibrous material. I have no idea what the shiny after-market pads are made of - could be foam under the metal foil.