I know some have already said their was no benefit to running a cooler stat since it is computer controlled, :?:, but if their was no benefit, you wouldn't have the option to retune for it, nor would people do it. I'll bet if some of these nay-sayers would try it, especially for racing, they would see a difference in ET's between running with a cooler engine and with a 190+ heat-soaked engine. My tuner suggests no lower than 170, so, I say go for it! My 2 cents.
The reason that below 180*F is not recommended, is also that the engine tolerances were engineered for the stock 192*F thermostat. (pistons, rings, etc) Tuning will add more timing and keep the A/F in check, but this still won't add much benefit to a stock motor.
On a basically stock car, running a lower thermostat without tuning for it isn't going to make much of a difference.
I run a 180*F in my car since it has a blower and I want to keep it cooler, but I also run the low speed fan at 174*F and the high speed fan at 180*F and am tuned for the 180*F thermostat.
The main mistake most people seem to make is that they think putting in a lower thermostat will keep the engine cool in hot weather. The thermostat will open at a lower temperature, but in hot weather, the radiator still rejects the same amound of heat and unless it is upgraded to a more efficient style radiator or the cooling fan set points are lowered to help reject heat, the car still runs about the same temperature.
If you upgrade the radiator and/or lower the fan set points, then you can see colder temps.
I had an 89 LX 5.0 / 5speed that came with a 160*F thermostat, it ran like crap, the heater didn't put out heat and the CEL stayed on. After I replaced the thermostat with 180*F thermostat, all the issues went away and the car ran like a champ. I will admit the newer 4.6 is more tolerant of running cooler.
Also we are for the most part talking about a daily driver not a race car.