Walt,
I mounted my SD-8 accessories exactly as you described, on the firewall of my RV-8. I have no way to tell you what the temps are there, but I have had no problems with the system after four months and 120 hours. I personally think the temps are pretty reasonable on the firewall, since the really hot air should be captured by the baffles and directed down and out, but that is just a guess.
At any rate, one data point is...no problem!
Paul
__________________
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 -
N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com