If you are stating that the higher ability of aluminum to dissipate heat from the combustion chamber is nominal. I'd bite on that. But there has to be more to it then.
My guess.. would be aluminum heads has better chamber designs. (smoother, shape, quench, etc) combined with engines equipped with these heads often having blueprinted deck heights, ideal cam specs, headers and even piston coatings affecting the engines compression/octane friendly traits.
In other words, while I think aluminum heads are more compression friendly, I don't think it's as simple as solely changing the material of the head that equates to this
