My question is...even if the head rejects heat quicker, does the medium used for coolant accept heat any quicker, or does the coolant not know (or care) what they head material is?
Largely true. If anything, think of the metal as a electrical conductor. How many ohms (resistance) in each one? If the head material is slow to move heat, then it will loose against the higher flowing, heat removing energy transfer metal.
I think the difference in the metals are realized in extreme environments more so than what goes on and talked about here.
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