A lower temp thermostat in my Focus did absolutely NOTHING. It will probably make it run cooler in the winter if outside temp is pretty low.
I checked mine with an OBD II scanner when it was 82 degrees outside with the stock thermostat. I changed to a 180 degree thermostat and checked again when the outside temp was 80 degrees. EXACTLY the same temp range. About 200 to 215 degrees depending on how I was driving.
My engine is apart right now for some other mods and when I put it back together I'm puttin the stock one back in. A thermostat doesn't set your operating temperature, the capacity of the cooling system does. Just my .02.
Guys, I think that the fans would need to be reprogramed as well to maintain a cooler temp. The thermostat may open earlier but the engine will eventually get to the same temp unless the fans are trying to cool it to a lower temp. This is what is recomended for my friends Z28. I'm not sure how or if you can change the fan programing. In his case he uses a hypertech reprogramer instead of a preprogramed chip.
i'm pretty sure that the thermostat will just make it think its slightly cooler. i was looking back over some older posts on this and no one really saw any form of gains. if you can get it for damn cheap, i'd say go for it. it might work better in your climate than others.
If you use your heat in the winter, don't do the change until summer. I noticed no difference in cooling or performance, only froze my butt off when it started into winter. Needless to say its was outta there PDQ.