Most thermostats are set in the 190+ degree range. There are a lot of opinions about better horsepower output if the temp is a little lower. I don't know how exactly correct any of them are, I'll leave that to others. But in the racing world, it's pretty commonly done. Of course, that's tracks, where you're driving hundreds of laps and pushing the engine really hard the whole time. It might be done in drag racing, too, but the impact there would be minimal since you're only running a few seconds.
A lot of old hands like to keep the engine temps a little lower when an engine is working really hard, to keep the oil from breaking down so quickly. Some would be wanting a lower temp thermostat for a truck that does a lot of towing, for example. Lower thermostats can also help with knock, especially if you've advanced your timing a bit. They do tend to interfere with fuel enconomy, though. Some guys swear by them. Some swear at them. Most modern engines don't have much need for them unless you're doing some more extreme stuff with them, running higher octane, working them really hard.
I'd have to be really working a truck hard all the time to consider it.
