Keep in mind that this is an infrared energy scan image in false colors, adjusted to enhance the contrast in a particular energy range. The only thing that pretty color image is showing you is emissivity, or the amount or radiant energy. The coated part may have emissivity of 0.2, while the uncoated part may have emissivity of 0.6 (+/-).

The coated header radiating less energy may actually be hotter on the surface due to more retained energy. If you put a temperature probe directly on the surface of the coated part it may actually be hotter than the uncoated part. The idea here is to keep the heat in the exhaust and push it farther downstream before it is released.

This does not cool the engine, and should make no difference at all in coolant temperature, since this is all happening after the exhaust has left the engine. It could make for less heat in the engine bay, which may be good for carburetors and modern alcohol containing fuels that do not like heat. Cooler engine bay may also drive less heat into the the passenger compartment.

However, emissivity is not the whole story. There is heat coming from the radiator which will not be changed. There is also a lot of heat coming from the exhaust manifold or headers as conducted energy. If you hold your hand near the manifold you can feel radiant energy. If you actually touch the manifold you will definitely feel conducted energy, and you will get a better indication that emissivity is not the whole story. The manifold is dumping a lot of energy into the surrounding air by conductivity. Since the coating is very thin it will have very little affect on conductivity. If you could know the actual amount of heat expelled by conduction and radiation, you may discover that the reduction achieved by the coating may be a lot less than you first thought.

I'm not saying that the coating is a waste of money. It can look pretty and it may reduce under bonnet temperature a little. For sure it will do nothing to cool the engine, so you should see no change of indicated coolant temperature. But it can help cool the carburetors, and may help cool the passengers.

Now if you also coat the inside of the manifold or headers, you can enhance the affect. The flip side of emissivity is absorption, or the character of accepting radiant energy. Again you need to compare this with thermal conduction to get the whole story, as conduction will not change much. But to the extent that you can reduce absorption of radiant energy from the exhaust gas, you can actually reduce temperature of the manifold. Again the idea is to retain energy in the exhaust gas and push it farther downstream before it is released.

Bottom line is this coating does nothing for internal engine heat, or heat from the radiator, or heat from thermal conduction. It only works on radiant energy absorption and emission inside and outside of the exhaust manifold. The effect of the coating may be relatively small compared to all other heat transfer going on, but it may be minimally useful to reduce engine bay temperature (if you can justify the cost).