Here we go again. The faster the heated water goes through the radiator, the water would not be cooled as much, as a slowing flow of water would. So, the water reentering the engine would be hotter going back in, making the thermostat open longer to cool the heated water. The reason for a three core radiator on an AC car. Allowing the heated water to be cooled longer. So, my thinking, slowing down the flow should help, somewhat, but not as good as a larger radiator.