To illustrate how critical it is that the thermostats shut the water flow completely off to maintain the water temperature, the stock thermostats actually have a check valve in the burp hole that opens to let air through.
But closes when water tries to come through.
That's the one weakness of the Stant stats - no burp hole. I drilled a 1/6" hole for such which may account for the minimum temperature being 10° below the stat's rating of 170° on the graph. But, the Shewman stats allowed 40° below their rating of 180° in this test but 60° below later in the day on a longer drive.