Does anyone know why they want to get a cooler t-stat? Do you know why you want your engine cooler?
The entire colder t-stat idea is to prevent KR. The colder the engine is, the less KR you should get. But if you mod correctly and tune your car, you should not have any KR to contend with. Thus you should be able to run a stock t-stat and get better mileage and better performance.
My personal opinion is stick with a stock temp t-stat.