My guess is that if you were cruising at highway speeds long enough, your coolant temps would have eventually fallen back toward 172 degrees. it takes a while in hot weather. Temps with a stock PCM will often creep higher than 200 degrees at low vehicle speeds because the fan is not set go to full speed until you are well above that mark. You just aren't getting enough airflow through your radiator to get down near your t-stat opening temp, even in winter.
As WGO alluded, not all Stage II flashes are the same in the fan department. My Boomer flash has the fans come on at about 185 but not reach full speed until 200 degrees, where my coolant temps top out. I run a 180 degree t-stat and bottom out there. The DC flash apparently has the fans come on full by 185 degrees. This keeps coolant temps lower but works the stock hydrostatic, power steering pump driven system a lot harder. And you get more fan noise. It also can make it more difficult during normal driving to get your oil temperature up toward 200 degrees, where it really should be for condensation burn off and best performance. Of course a few full trottle runs up to redline will solve this issue.
I honestly think that if you want to keep coolant temps lower the best approach is to install an electric fan setup. Why we don't have one from the factory is a bit of a mystery. However, lower coolant temps are not all that they are cracked up to be. 200 degrees is quite normal, and even has its benefits. What you really want to do is lower intake air temperature, which dropping coolant temps only does indirectly. Some guys go to methanol injection, especially with FI setups. I think a composite intake (with less heat sink) from the factory or the aftermarket would help a lot.
I can understand your frustration with the thermostat. Especially as it's a &%$#@!! to install...
Just my $.02.
