Remember, we are assuming our car is overheating. So think outside the box a bit and I'll try to walk us through this. Lowering the temp at which the fan starts, or thermostat opens doesn't change the cars operating temperatures unless you set them above normal operating temperatures. Even then, your engine will try to finds it's groove. Setting the fan to come on, or the thermostat to come on at 150 still means a car that operates at 190 will still go right to 190. Better transfer of heat is the only way to make a cooler running engine. Fluids that transfer heat better, or finding ways to increase the flow of fluids can change things. Even then. if water is passing through the engine faster than the the radiator can cool it, then you might still have a problem.

Then there's air. More air flow. IE, a bigger fan, and or radiator. or creating areas to pass more air through the radiator. And plenty of room to allow to escape can cool an engine down.

The third thing is that a system that isn't boiling over is not overheating. It may not be cooling well, But until it fails, it's working. Which brings me to the last trick. Raising our boiling temperatures. This can be done with Coolants or raising the systems pressure. The higher the pressure, the lower the boiling point. But you can't have your cake and eat it here either. To much pressure causes other failures. We aren't building steam engines. Higher coolant ratio's are bad too. Water transfers heat much better than coolant In the end, it's a symphony. It's a system where any weak link in the chain can wreak havoc till you find the culprit. Ask any Street Rodder...

Remember, the thermostat is only there to allow the engine to get up to temperature faster. Our electrics fans do the same exept they actually cool, and moderate the engines temperature. If your car overheats when the fan is running, what temperature it started running at is immaterial. Your just masking a failure.