When I removed the factory heat shields from the exhaust I used a self-adhesive aluminized fiberglass cloth to cover the underside of the trunk. It seems to have worked pretty well in that application, the trunk stays much cooler than it did from the factory (but there is a lot more airflow now that the factory insulation is gone, I'm sure that helped). Here is what I used:
I did the same as well. During my REV400 upgrade with the S/C off I noticed how dingy that carpet was (it was filthy). Cutt it all out, cleaned the firewall, and applied DEI Reflect-a-Gold 12x24 sheets. It was a complete pain in the ass, mainly just really time-consuming, to do even with the SC off.