It's a little misleading saying it like that... In order to maintain the heat in the chamber of an aluminum head at the same level as an identical iron head you NEED to run more compression necause the aluminum conducts the heat to the cooling system much faster than iron. So it's not that you can run more and get more power... But you run more to get the same power. Now by using the better chamber shape and the right piston you also get benefits from detonation prevention... Which means less octane in the fuel for that same increased compression. It's all about the parts working together. On a properly designed engine package either head material will make similar power. You just build to optimize the benefits of whatever you're running.
