Mk3 Thermostate Question

[f00g00]

Running a lower temp thermostat will not make the car run at that temp. It will still run at normal operating temp but the lower thermostat will probably just let it take a little longer to warm up.
The reason your getting more power is cause your running a little richer fuel mixture while its still warming up. Is that what you really want? If your running rich all the time then your fuel economy will suffer. But if you must have this extra power all the time without playing with your cooling system there is a mod you can do electrically that will get you what you want without sacrificing the cooling system efficiency.
You can read about the mod here.
http://www.supras.com/06/techcenter/display.php?QID=23
I tried it years ago but to try and make mine run leaner for better fuel economy. That was before I found out that leaner on these cars is a no no.