Running a colder than stock thermostat is a bad idea for a number of reasons. For one, your ecu determines if your car is in warm up mode by the coolant temp. By running the coolant colder than it should, your car will always be running in warmup mode, which dumps in fuel and changes timing. It is just a recipe for less performance and poor mileage.
Second, it will make your car take forever to warm up, especially in the winter.
I say all of this from experience. I used to run a jrsc 160 degree thermostat on my setup and it caused all the problems I just described.