andrewr, you have to remember that heat transfer is a function of temperature and temperature in a piston engine is a function of pressure, so you can dramatically shift the heat energy flow by raising and lowering the pressure by shifting the Pmax point with ignition angle or burn rate.
On an aero engine, ignition angle is fixed. The ignition delay is variable with air-fuel ratio as is the burn rate, so air-fuel ratio will have a large influence on the magnitude of and angle at which Pmax occurs, which in turn influences whether the heat goes to the cylinder head, makes mechanical work or heads out of the exhaust.
Oh, and exhaust valves burn when they leak. That's pretty much the only way you will overheat them without melting the cylinder head at the same time.