Okay, some simple physics. The amount of cooling that the radiator can provide depends on the difference in temperature between the water and the air going through the rad. If the water stays in the rad longer it will get cooler, but the amount of heat given up by the water will be less because the temp difference is less. Having a higher volume of water through the rad increases its cooling capacity. Having a restriction before the rad makes for more pressure in the engine to allow a higher boiling point on the water but slows down the flow. Increasing the efficiency of the water pump provides both higher flow and higher pressure. Increasing the flow of air through the rad improves cooling as well by increasing the temperature difference between the air and the water.

The engine will be cooler for longer without a thermostat but if the rad isn't big enough or doesn't get enough air the engine will still overheat. It's better for the engine to get it up to operating temp quicker so I always run a thermostat on the street and sometimes on the race track depending.