Well Monstar, I have no empiricle evidenced perse. But. My tuner recommended it and it stood to reason. The engine runs a few degrees cooler. My tuner reckons with the cooler thermo, they make a bit more power at operating temp. It's logical to me.
My Engine Coolant Temperature (ECT) sits at 87° never gets above 96° ECT while in operation (sometimes after sitting). That’s because of the fuel and good combustion impacting indicated thermal efficiency. Acording to GM 85-87° ECT is about as low as you want to go for engine damage (wear), fuel mileage and power.
ECT is the fundamental factor for engine tune, oil spec, fuel type, service interval / duty, HVAC, transmission, and ultimately vehicle speed.
In general running at 72° vs 87° means the ECU flags the engine as not ready, compensates and so affects the tune. There is no good reason to run your engine colder with tighter clearances than engineered tolerance, unless the engine is re-engineered. Also Unless your engine does currently and consistently operate at stable 87° with factory tstat then lowering the ECT min with cold tstat *may* result in colder stable operation but likely you are just going to broaden the heat cycle range. This in itself has nothing to do with increasing thermal efficiency.
I have a 160° tstat in a drawer because I noticed G8 drag racers on gasoline do that. The benefit is to extend the effective range of the cooling system and start their run at a low ECT with the aim of precision windage at the expense of increased friction, reduced engine protection, and momentarily slightly lower IAT.
Results are inconclusive and introduces more variance than predictability.