Just one data point
I've had my RV-6 for 23 years (2000hrs). During all that time I've had a time bomb (aka crappy insulation) on the cockpit side of the firewall.
During the last annual, and due to all the good work that Dan has done, I removed it all. I had forgotten this, but I had put down a good quality tape (non adhesive transfer used in composite layups) on the firewall first, and then adhered the crappy insulation with hi temp silicone. It came off in chunks, but once the tape was peeled up it left no residue on the firewall.
For fun, I put a match to it....oh boy did it go up in flames nicely.
For the data point, I've flown it since and I can't tell (not very scientific by ANY standard) any difference in cockpit temperature or noise level.
I am using Bose headsets, so I'm sure that really helps for sound.
So IMHO, you don't need anything on the cockpit side of the firewall. Dan has made a good case for fiberfrax FORWARD of the firewall, and you can find the thread here:
https://vansairforce.net/community/showthread.php?t=72087
Regards,
Laird