Yesterday I took the car on a pretty long drive, mostly 40 - 50 MPH, up and down hills, not on the highway, to a car cruise. The outside temp was 89 degrees. The last part of that drive was idling the 3/4 mile cruise route (starting and stopping, people walking out in front of the cars, the usual stuff). I got up to just under 200 degrees. I previously would have been at between 210 - 215 degrees with the 180 degree thermostat, with no other changes. I'll take it, glad to be running 10 - 15 degrees cooler just by changing the thermostat. I'm assuming that it's a combination of the thermostat opening temperature and the high flow, that the thermostat is less of a restriction to overall cooling system function.
To all the theoretical discussion above, the purpose of the cooling system is to prevent the car from overheating. My GTO cooling system capacity was 19.4 quarts according to the manual, could be slightly more with the Desert Cooler radiator. That's the volume that's available to circulate to dissipate heat. If you could run a huge 10' x 10' radiator, of course it would allow the system to run cooler because the volume NOT in the engine block at any point relative to the coolant volume in the block would be much greater. It's the same idea that if the ambient air temperature is 10 degrees, the system's going to be able to dissipate more heat and run cooler than when the temperature is 90 degrees. Ambient air temperature matters, unless you have a cooling system with much more capacity than is required.
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