Originally Posted by
Cuda340
Colder thermostats have a place in hot rodding when significant power increases, most often from adding a supercharger, require greater reserves for momentary overheating. But in a stock or near-stock engine, the design operating temperature yields the best fuel economy, emissions compliance, and engine life."
I think this is the key part of the equation. If you have made radical changes to the operating environment under the hood, you may need the extra cooling. If the car is near stock, it will probably be most efficient at or near the intended operating temperature.