[mpiper]

Basically, a cooler thermostat allows the radiator fluid to START flowing sooner, and then has a slightly higher flow rate. It doesn't really add any power, it keeps the car from getting as hot and all the related side effects.

A cooler car has less chance of detonation, so you can run more advance. A cooler engine means a (slightly) cooler engine bay, so a bit denser air going into the engine. A cooler engine also prevents premature oil breakdown (not as important with synthetic, but still happens over time) meaning the oil/engine should last longer.

That's about it. Does it mean more in hotter locations? Possibly, but once an engine starts running, the engine bay temperature goes pretty hot pretty fast. So it may not be as important in 10 degree weather as it is in 90 degree, but it will still benefit some.

Of course to cold of a temperature outside (below 0, -10, etc.) and 160 might mean the engine cools too much and never gets to best operating temperature. It really depends on how well your engine bay flows air and cools off when moving.