Now there has been a lot of controversy back 20+ years ago regarding engine coolant temps.
Back in the day (carburation), everyone wanted to run at the coolest temp (160-180) and it was the common thought that the cooler, the better.
Unfortunately, that isn't the case. They mistakenly discovered that the hotter the engine, the better it performs (within reason), not just in emissions, but actually power and efficiency. Even with the engine having a carburetor, the warmer it is, the better it will run. Now, there is a fine line between too cold or too hot. Generally around 190-210* F is ideal, anything below, it will become inefficient. Anything above, youll start having overheating problems and exceeding the limitations of the coolant.
An engine running at or around those temperatures is the ideal temp for combustion and fuel atomization.
Now, with the thermostat, it does NOT primarily determine normal running temperature. It all falls into the cooling system itself. Radiator, fan, hoses, coolant passages, water pump flow, etc... A thermostat determines the "fine tuning" of the temperature (within 5-10*) and proper main flow rate. If a vehicle is run without a thermostat or an incorrect thermostat, this may cause more damage than good, in fact it may cause overheating. The coolant would be flowing too fast for proper heat transfer from the engine to the liquid, in turn, keeping the heat inside the cylinder and cylinder head (where all of the heat needs to escape). Plus, this a electronic fuel injection system. ALL electronic fuel injection systems (and some mechanical systems) use coolant temperature as a PRIMARY ECM input to determine fuel mixture. Too cold, it adds fuel too warm up to "normal operating temp". So if you have an engine designed and tuned to run at 200*F and the coolant temperature is at 160*, it will add fuel (richen the mixture). So, if its like that constantly, you will have a drop in fuel efficiency and potentially, after a while, throw a fault code and the vehicle will run at a default setting.
If you install a 180* thermostat in place of a 200* thermostat, at operating temp, the thermostat will open more than the factory thermostat would Causing a higher coolant flow rate, therefore potentially causing the above stated problem.
"God created Police Officers so Fireman could have heroes!" - My late Father