A trick thermostat won't fix a junk water pump or other cooling system component issue. My 13' has only gotten hot on ice with stock cooling system, along with many others out there. Unless I'm misunderstanding, Dave's T-stat addresses power fade from a mild heat soak issue, not full blown over heating.
My thoughts: Does the cooler get hot? Does it puke coolant out of the overflow? Is the engine itself getting hot or does it seem more like a sensor issue? Have you run the engine with front end up on a ramp, with the pressure cap off, to get air bubbles out? Does the cap seal to the bottle properly?
An IR thermometer can be useful in this situation to monitor temperatures.