Mopar Action SB head article
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.
Here's the problem, all things being equal then the iron head motor would make more power, not that much more though and since all things are not equal the advantages of an aluminum head far outweigh the little bit of power that is lost to heat dissipation of aluminum. First since AL heads do dissipate heat faster than iron you can run much more compression before detonation, thus more power. AL heads flow better than iron heads. All the high flow race heads are AL, you'll never find iron race heads that can flow with the AL heads. They are lighter and 40 pounds on the nose of a race car is not something to sneeze at. And again they are easily repaired.
So which really is better? Well let's see, is there any NASCAR, Busch, or ARCA team, car or truck running iron heads?
No I don't think so.
How about Pro Stock, any NHRA or IHRA car out there with iron heads?
Hmm, can't think of a single one.
Any one in Pro Mod run an iron head, or any other class in racing where an AL head is allowed?
None that I can think of and certainly none of the top cars.
So while theoretically you are correct, real world reality is AL makes more power.
You can always tell an engineer, you just can't tell him much. :)
(that's a joke)