So, this is not totally confirmed. My 1776 with 9:1 static compression has run flawlessly climbing up to 10,000 ft elevation in year round 85f-90f temps with less than the full engine compartment sealing.
I measure head temperature at the exhaust manifold bolt continuously with the Speeduino ECU and never see temps above 270f. Highest engine compartment temps I measure are 155f at total heat soak.
I have the full complement of engine tin including sleds under the cylinders, but the edges of the tin are not sealed to the body. I have custom fitted stainless and rubber seals made, but I just didn't reinstall them after an engine revamp 18 months ago.
I have a large rectangle cut behind the license plate, but the firewall is solid. The engine lid had the extra Carrera style cutouts and there is no rain tray to impede airflow.
Note, this isn't a high horsepower engine with a lot of displacement. It's a modest engine built to be frisky, but to also last a long time.
I suspect if you have a bigger engine, the rule about total sealing holds. I figure mine is more like one of the dune buggy configurations where you want the essential engine tin, but a lot of the engine is just hanging out in the airflow.
I wired for three engine compartment temperature sensors recently and haven't had a chance to do the research on air temps in various locations under various conditions. Just wanted to say that it's not a given that you HAVE to do full sealing to avoid temperature problems. That said, it's not that hard and we know it can only improve things.