Anything that significantly raises the engine temperature is a bad thing. If it really raises the temp by 40F, I'd call it a bad thing. An optical pyrometer pointed at the engine surface is measuring the temperature of the metal. It's not measuring the temperature of the air. There is also a temperature gradient in the engine. The internal engine parts are hotter still.

The best analogy to understand heat transfer is electricity.
The temperature gradient is voltage.
Heat flow is current.
The metal is a resistor.

If an engine is making power, it's generating waste heat (over 60% of the energy of gasoline is waste heat). The idea is to get rid of that waste heat and minimize the temperature of the engine. Adding paint is adding another resistor in the circuit.