I installed a 180° stat in my previous truck, while it ran slightly cooler on the highway, the fuel mileage plumeted and their was zero performance increase. Modern engines are designed to run a hot combustion chamber to completly burn all the fuel maximizing power. Unless youre tuned for a 180° t-stat and by tuned, I mean more than just changing the fan setting, its not doing anything.
If you're lowering the water temp, you're going to lower the cylinder head temp and thusly the chamber temp which is partially why you get the loss in fuel mileage (the other reason is the oil temp as most oil lubricates best, less frisction, around 200°). You'd need a hotter range spark plug to get the chamber temp back up and not hurt performance. A cold cylinder isnt making power despite what anyone thinks