I fly my 14 in south Louisiana, where the temps this summer were often 95+. I never open the heater flaps, and fly with socks on no shoes. It's pretty hot taxiing around but I kept the canopy cracked until takeoff; I usually latch it for the runup it wants to come up some when you rev the engine.
With all of that I haven't found the firewall to be a hot problem. I don't have any special insulation on it. Now that its colder I'm especially not noticing it in any way.
My noise cancelling headphones do pretty well; I have the Bose and the latest David Clarks; the noise cancelling feature of the DC mic make it the winner for me in this plane.