More heat into the coolant would mean more fuel burned too. OEM's all using aluminum heads suggests that there must be other reasons to use aluminum (hint: it's cost and weight savings).
Not at all. (Though I won't argue at all with that being the OEM reason to use it ...)
If the head chamber surfaces are truly cooler, it is because the thermal resistance of the heads is lower, and thus overall thermal resistance from gases to air are also lower. That means the heat flow through the heads and through the coolant, through the rad, and into the air will increase..... because the whole heat flow path, from chamber gases to outside air, has a lower overall thermal resistance. But the thermal resistance from coolant through rad to air is the same. So the higher heat flow raises the coolant temps.
I know it seems wrong to say that. How it changes:
Temp drop from gases to head material is higher (which absolutely says that heat flow will be higher)
Temp drop through the head itself is lower (due to much lower thermal resistance despite the higher heat flow)
Temp drop from head to coolant is higher (due to the higher heat flow)
Temp drop from coolant to air is higher (due to the higher heat flow)
There are a whole lot of 'all things being equal' in there.....like coolant flow rate, and surface finishes, which have a big effect on thermal resistances.
