well heres a theory. on you big tune, yoour run it hard. This consists of big fuel, 45psi of boost and WOT. thats creating alot of heat in your heads. If 1 or both thermostats are not opening all the way, i can see you building some more than normal pressure in the cooling system as the water is getting hotter and not being able to be cooled by the radiator. The stock temp gauge is not accurate. With my EFI Live hooked up pullin a trailer up a 6% hill in 100* heat at WOT, near the top of the hill EFI Live said i was at 230* and the factory temp gauge was at 205-210.
If you dont pull anything heavy nor romp on it all the time you may never see high temps under load from a sticky t-stat, specially since we have 2.
You may hook up EFI and got run it around and see what it says your temps are at on the big tune and WOT a few times