I run a 180* thermo just because it was laying around. I don't notice anything. Boosted cars need lower temp thermo's because the extra heat and load the engine has to deal with. I think the stock thermo is 192*. I hear 4.6's like to run on the hot side though.
Modern engines are designed to run hotter than their earlier counterparts. I beleive the lower t-stat trick is just a hold-over from the carb days. Running too cold on an EFI engine can have drawbacks. I think you should just leave the stock t-stat in there alone.
The lower t-stat was a trick from the carb days to be able to run more timing & run richer... and on boosted FI applications, its good to aid in the prevention of detonation. Not worth running anything less than the 196* t-stat on a N/A street driven car. Gas mileage will get worse the lower you go. More wear and tear on the internals with a colder t-stat on our cars too. These things are designed to run hot.