re: "...Same principle, as far as I can tell, as the "increase efficiency" of installing an electric fan in front of the engine. But that one still has its supporters...."
I haven't put an electric fan in front of my engine to replace the auxiliary belt driven fan (although I have an auxiliary fan for the A/C in my '93s).
But the reason (or perhaps rationale, at the least) for it's claimed improved efficiency is simply that it isn't driven as much (i.e., as often) as the belt driven OEM setup.
Even with a good fan clutch, there is still some parasitic "drive", as seen by a running, though stone cold, engine (even with the car stationary) having its fan turn. Ideally, it shouldn't begin turning at all until the radiator warms up to (in turn) warm the fan clutch, but I've never heard of a clutch that efficient. Thus, the fan extracts a parasitic drag on the engine at all times, although more when the engine is hot than when it is cool.
Compare that to an electric fan, assuming it has a thermostat linked to the radiator or its intake hose. A stone cold engine can run and demonstrate that the fan is not yet turning, unless the car starts moving through the air so that the fan turns passively as a turbine* (though not as a propeller*), so there is no parasitic drag on the engine until the fan is switched on in order to enhance cooling (usually only when the car is going slow or stopped in traffic).
[ * clarification: at least to me, the word "turbine" implies a device that is caused to turn by a fluid passing through it; a "propeller", in contrast, is a device, driven by some force, that causes a fluid to flow through it.]
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