Opening up the radiator cap with an electric water pump shows a whole lot more flow at idle than a mechanical pump.
And the relevance of this observation is what?

Remember, typically these electric water pumps only present one speed, so that, that you witness at idle (that with the exception of an increased pumping performance as the voltage increases with alternator speed and output) is all your gonna get, any where, at any time, and besides, who needs max-flow at idle? And since these pumps are not effective at producing pressure, this in a system of passages engineered for a certain sum of pressure to force the fluid the distance to be traveled and about the obstacles presented, the flow rate specification expounded by the manufacturer actually turns out to what otherwise is often termed as a "free-flow"sum, wonder what the flow rate is in the real world as installed?

Perhaps for a more relevant flow comparison, now with the mechanical pump, remove the radiator cap, accelerate the engine R.P.M's, and in some applications I would suggest you might want to stand back a little, now which pump is actually moving more water?

The mechanically driven pump provides for a variable speed and hence pumping rate, less at idle and low speed (R.P.M.'s) when the engine is producing less heat needing to be removed, and then as engine speed increases, and most often also the load, and associated heat, the pumping rate and pressure is increased (it's like magic

) as is required for reasonable temperature control.
Also remember that the engine does require a certain sum in elevated temperature for proper function, so how does a single speed pump work effectively on a variable B.T.U. production engine?

"Thermostat", the man says, but these devices are designed around a variable orifice restriction and a pressure differential function, but remember the electric water pumps generally do not provide such, so this established engineering function is skewed; this meaning in actuality, we're just "lucky" if it all works, or if works at all! But even then, how much of this might be perception versus reality

So. how about maybe radiator "shutters"............................

Scott.