Well... I'd say the answer to that question depends on the heat load vs the cooling capacity. If you are driving down the freeway with a low load and high airflow through the radiator, you will likely see lower temps. But if you have low airflow, a cooler t-stat won't do anything for your coolant temps as they will be controlled either by the balance of airflow vs heat input, or the fan setpoint (which increases air flow and thus cooling).
I've read that the drag strip guys love the lower t-stat and fan option because a lower coolant temp results in higher HP, but as you note, I'm not sure what impact that has on daily driving air fuel ratio's.
I do have my fans set to come on earlier than stock, but my car needs as much heat as possible removed on the track, so the fans help. :cheers