It's not a myth. But, what's normally lost in the shuffle is that closed chamber designs of either material are much more detonation resistant when built with the science of quench affect than open chamber of either material. A properly designed engine takes into account the properties of the head material and the mechanical advantages of the chamber shape. So while aluminum does take more heat out of the chamber and reduce surface temps in the chamber, a good builder will just add more heat and make more power.
Your question would really be pertaining to an engine that was poorly designed from the get go, and is now looking to crutch it into less tendency to detonate. In that case it may be better to address the other causes of too much cylinder pressure that are cheaper and easier - i.e. the camshaft.
