[KMag]

The Carnot efficiency does drop as T_c increases, so all else being equal, for the same amount of fuel being burned, you'll get less energy. However, more importantly, at higher temperatures, a given volume of air at the same pressure contains less oxygen. All else being equal, at higher ambient temperatures, you're burning less fuel. For a turbocharged engine, you could increase boost to counter-act the reduced density, but that increases both mechanical and thermal stresses within the engine.

Anyway, I think the reduction of oxygen density is the primary driver for reduction in power as ambient temperatures rise.