No myth just plane simple physics. The aluminum sucks away more heat than iron, less heat means less cylinder pressure which means less power.
Why do you think they tell you run more compression with aluminum heads? Its to make up for the lost heat do to aluminums ability to transfer heat better than iron. This is also the reason they put coatings on chambers and piston tops, to keep the heat in the engine for more power.
When you burn a gallon of gasoline it has the potential to release a fixed number of BTUs of energy. Its that energy in the form of heat that pushes the piston down the cylinder. A 4 cycle internal combustion engine is not very efficient. Some of the heat is used to push the piston some is disipated away into the metal of the engine and the rest goes out the exhaust. Anything you can do to keep the heat in the cylinder will make more power.
