if i understand thermostats correctly, they are not fully open until 10-15 degrees above their rating. in other words, a new car stat coming with a 205 degree rating will not reach full opening until 215-220 degrees.
if i understand heat transfer correctly, the larger the difference in temperatures between the coolant and the surrounding material[s] [the radiator structure], the better the heat transfer, thus the better the cooling system will operate.
back in the day, a common factory T-stat was a 180-195 unit, so it is conceivable it was not fully open until 215 degrees.
i too, used to be a believer in keeping temperatures in the 160-175 degree range, but i have had much better running success utilizing higher temperatures than lower ones, so that is my preference now.
still, there is more to it than just placing a piece of cardboard in front of the radiator to bring the temperature up.