Here is an easy test you can do. Get both a 180 and a 195 thermostat, boil some water in a pot on the stove with a thermometer in it, and put both thermostats in the water at 220F. At that point you should be able to clearly see how far both are open without taking apart anything on the car. I suspect as you said that both will be fully open well below 220F, and so I suspect switching to a 195 will not improve anything, but if the stovetop test shows otherwise, then it may be worth a try. You can also test at other temperatures to see exactly when they open, how fast they open, when they close, etc.