Putting a lower thermostat only means that coolant will flow to the radiator earlier than normal, actually making the car run slightly less efficient. Engines are most efficient at operating temp and getting there quicker is better, but that's just pennies. You can do it or not, but it likely won't change anything once the car is fully warmed up.
Water wetter additive will probably have more effect than a Tstat. The best effect you can probably see is from an aftermarket radiator with more capacity. If you're able, try to get a second opinion from adding another gauge, preferably an old fashioned mechanical one somewhere visible in the engine bay.