The -10 degrees hypothesis doesn't hold up. You could run a car without a fan at -10 and it would likely stay at the rated temp on the thermostat. A 100 degree change in ambient air temperature is going to affect the performance of a cooling system on any car.
The thermostat manufacturers state that the range from "begin to open" to "fully opened" is 15 - 20 degrees. They're designed that way. I'm assuming that if you have a 180 degree thermostat installed and you run 195 - 200 that's OK; if you're running 220 you may have an issue.
I have a 180 in mine now, I'll run up to 195 on a hot day, it may get to 210 on a long highway drive on a hot day. I ordered a 160 to try it out, we'll see how the temps change with no other changes to the cooling system once it's installed. If I'm still at 190 - 195 with normal driving, we'll know that the thermostat isn't the determinant in my car. My car's an A/C car, I have a Desert Cooler radiator and the factory clutch fan setup, shroud, etc.
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