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The point of bottle-necking the cooling system with a thermostat is to create higher than normal atmospheric pressure in the cooling system. Higher pressure helps raise the normal boiling point of the coolant so that it can carry more heat away from the block. Notice that the bottle-necking only occurs before the coolant re-enters the block and obviously after it has circulated through the radiator. The rest of the way, it's free-flow till the coolant ends back at where the thermostat is. The radiator cap also helps with creating higher pressure (and more so) as the spring in the radiator will not let any of the coolant move to the overflow reservoir until the cooling system reaches a certain pressure.
If you add the higher pressure system to a larger capacity radiator, you are enabling the coolant to spend more time in the radiator to help shed away even more of the heat it carries from the block compared to a stock cooling system.
Even if the coolant's normal boiling point may be 215* (unless it is pure water), under the pressure created in the cooling system the boiling point now would be around 220-225*, depending on how high the pressure is.
So you want a little bit of bottle-necking and a higher-pressure capacity radiator cap so you can actually help the block get cooled better.
The block has to stay at a certain temperature, which is why the need for the thermostat. That's why the needle on the temperature gauge shows the coolant being colder than normal when you have no thermostat in place. The various ratings of the thermostats help ensure that you reach the maximun highest temperature goal that you have in mind. And when that doesn't help, then come fans to help reach whatever highest temperature it is you want it to be at.
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