This might be a poor choice of words. If there is "more heat flow" then you are suggesting there is MORE HEAT AT THE SOURCE. Has something changed? Is there more "fire in the hole?"
If the "fire" (heat) is the same and there is "more flow" into the cooling system, then WHERE DID THIS HEAT GO previously?
It HAD to go somewhere. Did it go out the exhaust? If not then the engine as a whole had to absorb it radiate it and this is not magic.
Not a poor choice of words; it's a reflection of what is really going on. My words 'more heat flow' meams more heat flowing through the cooling system due to the AL heads. So, it's not more heat at the source but a change in where that total heat is flowing. And yes, the 3 paths are through the cooling system, and out the exhaust, and out of the engine's surface.
All of the heat transfer from combustion gases to heads is by conduction due to direct contact between the combustion gases and the head surface (duh). (And incidentally, that is why we polish the inside of the combustion chambers and eliminate high spots; the smooth surface has less total surface area than a rougher cast surface, and the head transfer to the heads is lower due to lower surface area, and thus head surface temps and the tendency to detonate both go down.) With the iron heads there is less heat flow through the heads and into the cooling system and more out of the exhaust (and perhaps a bit more from the surfaces), because the iron heads' interior surface temp is higher due to its higher thermal resistance; this higher surface temp prevents as much heat transfer from the combustion gases. The AL heads' can aborb more heat from the hot combustion gases due to their lower surface temp.
By far the biggest outflow of combustion heat is via the exhaust so a change in heat flow through the cooling system will not be easily measured in exhaust temps since the change in total exhaust heat content, and thus the exhaust temp change, will be tiny compared to the exhaust total heat flow. But the changes in cooling system temp are easy to see due to that last large series resistor in the heat flow path that you correctly ID'd, and which is called the radiator. For same radiator with the same airflow, fan shrouds, etc. (i.e., the 'all other things being equal' situation), any increase in coolant temps reflects an increase in heat flow through the cooling system. This is somewhat intuitive to all gearheads because we all know that as we load the engine more, the coolant temp increases, right?
I encourage you to look at it just like an electical circuit since you are so good at the electrical stuff. It won't lead you wrong; the analogy between DC electrical flow and heat flow is exact; the same Ohm's law relationships apply. We just don't have the equivalent of an ohmmeter and a current meter to measure thermal resistance and heat flow directly; we only have a 'voltmeter', called a temperature guage, and have to derive the other 2 by computation. That is not something one does in a garage, except by inference from a temp guage.
And speaking of outflow of heat via the exhaust, that is where we would like ALL of the heat to go. But the nature of the beast is that the hot combustion gases ARE in contact with the combustion chamber and cylinder and piston surfaces (again, duh), and so we have to have a cooling system to keep those temps in check.
Edit to add: BTW I did not read Moper's post above before writing and posting this; I wrote my post here, then took out the trash and hooked up a trailer and loaded a truck and them came back in for a break and saw and read his post and found the same thing being said. So it is 2 independent analyses...for whatever that may be worth.
