I think L18_B110 hit the nail on the head with the fan issue.
As for water pump. The pump is there for the sole purpose of pumping coolant through the engine. Since engine heat is in direct response to engine speed, it is natural that at lower engine speeds heat production is less, therefore less coolant flow is required.
The radiators job is to dissipate coolant heat as quick as possible. If it cant meet the demand placed upon it at maximum heat and flow. Then the radiators operation is insufficient. Everything below this temp is satisfactory, as the engines heat it governed by the thermostate.
Electric pump only has one flow rate. The radiators job is still the same and the thermostates job is still the same. A constant flow rate is only really suited to a specific engine rpm. Any rpm below this is just wasted energy on the pump and any higher rpm will mean an insufficient flow rate. So naturally electric pump flow rate will have to suit maximum engine rpm, to meet heat production demands.
Good for an engine that stays at a specific rpm. Any rpm less means less heat, thermostate reduces coolant flow through the engine and that reduced coolant flow places a load on the electric pump. Catch 22, more load on pump means more current draw, therefore more load on the alternator.
Power dont come for free. It takes more energy to produce a specific amount of energy.
Maybe there is a reduced load power gain with an electric water pump, but how little does it really equate to?

My bet is and electric waterpump was first used buy a drag racer who wanted to reduce engine loads by removing the fan belt, but still wanted a quick means of coolant flow on the return trip to the pits