A thermostat ONLY sets the MINIMUM operating temperature. That's it.
Once the thermostat is opened, the operating temperature is controlled entirely by the rest of the cooling system. If your engine runs hot, putting a colder thermostat in it doesn't change a thing other than the amount of wear you will put on your engine operating at a temperature colder than what the engine was designed to run at. Maybe you buy a little more time at a colder temperature, but if you're driving the car more than a few minutes the temperature will end up right where the rest of the cooling system is capable of keeping it.