Overdriven pulley how much is too much?

Not sure if it's been pointed out, but another thing to keep in mind is that heat transfer is not linear over time. Generally, the rate of heat transfer is higher with a larger temperature differential between the water and radiator or engine - meaning more cooling is done right when the hot water touches the radiator than when the water is just about to leave the radiator. The more flow you can introduce, the more energy will be pulled out.

Go with the high flow pump.