BB, here's my theory on this. The water pump has to flow some minimum amount of water when the engine is idling and the car is stopped in traffic on a hot day, so that the fans and the radiator can keep the engine cool. This is the worst case situation for the pump, and I believe that because whenever I've had cooling problems with a vehicle, it has been from sitting in traffic on a hot day. Once you are moving down the road, airflow through the radiator has increased and cooling is much more efficient. At higher engine speeds the mechanical pump is flowing a lot more water, but it is not necessary, and this additional water flow is being restricted somewhat by the thermostat anyway. Turning the factory water pump at the higher engine speed is not required, but since it is connected via a belt to the crank, it has no choice. Waste of horsepower. An electric pump only turns at the speed required to provide the necessary waterflow, no matter the speed of the engine.
Power in watts is related to power in HP like this: 750 Watts = 1 HP. You can calculate electric power by multiplying current in amps by volts. So, for an electric water pump that draws 12 amps of current, and is fed by a 14 volt supply, power in watts is 168, or 0.224 Horsepower to run the pump. Of course, in the car the power has to come from the alternator, and the alternator is not 100% efficient. Let's say it is 30% efficient in converting mechanical power to electrical power. This means that the alternator requires 0.224/0.3, or about 0.75 horsepower to drive the electric water pump.
Meanwhile, your mechanical water pump chews up about 10 HP at high engine speeds, and you don't get any better cooling performance as a result. Going to an electric pump is pretty much a no-brainer, IMO...