The thermostat controls the operating temperature of the system. The rated temperature is the temperature that the thermostat BEGINS to open up. It takes an additional 15-20* for the thermostat to fully OPEN, so a 160 stat is fully open by 180*, a 180* stat is fully open at 200*. An engine will not run cooler if the thermostat won't let it. If the temperature of the coolant runs more than 10-15* above the thermostat rating, the thermostat has lost control of the system. The radiator should have adequate heat transfer capacity to over cool the engine. As long as that is true, the thermostat will open or close enough to keep the coolant at the rated temperature. My philosophy is to get the biggest radiator you can, and then set the operating temperature with the thermostat. Aluminum radiators out perform brass copper radiators, that is why you only need 1 or 2 rows maximum. Aluminum radiators have bigger tubes, Brass copper is too soft to support tube sizes over 5/8". Aluminum can support tube sizes up to 1 1/2". More tube to fin content, more cooling. My nearly 20 year old Griffin radiator with it's 2 rows of 1 1/4" tubes keeps my 600 HP BBB engine within 5* of the 180* stat rating. If you have a 160* stat, and your engine runs at more than 180*, there is a problem with the cooling system, the thermostat is fully open and the maximum allowed rate of coolant flow is being admitted to the radiator, simple as that.