I'm looking for advice on how to repair/replace/improve the firewall insulation.

Everything is stripped down for addition of a "Dynamat" type floor covering and new carpet. At the firewall there is a special type of insulation with a heavy tar-board like skin. I am wondering if it would be best to pull it out- at least as much as I can access, and lay the mat under it and then replace it. I am not currently planning to pull the entire dash out for this project. Alternately, I could put the mat over this section or simply "build up" a replacement with layers of mat and insulation. Any advice here? My goal is to insulate for heat especially in this area as well as sound insulation. A lot of the interior heat seems to radiate from the dash area and I presume it is coming from the engine compartment.