It's not about the temperature of the head, if it was you could just make up the difference by altering the coolant temperature.

It's about how quickly heat is conducted away from the chamber and into the cooling system. Just some numbers as an example but lets say you have peak cylinder pressure of 1200psi and peak combustion temperature of 4000 deg F, obviously that's a huge delta between say 200 degree coolant. How could a material separating those 2 one of which multiple times more thermally conductive -NOT- drop peak temperatures and therefore pressure?

Combustion heat/energy that could be pushing the piston down is lost to the cooling system which is then dissipated by the radiator.

As long as the cooling system isn't on the brink of being overloaded you could easily have more combustion heat being wicked into the system (by a head with much greater thermal conductivity) while maintaining the same coolant temp.

If that was not possible you would have a very large swing in coolant temp at say idle vs high rpm wide open throttle.

Hey, at least we agree that HP is king