You likely won't see a difference in the engine temps, as this is a function of your cooling system... i.e. - if you have a 195 degree thermostat, your engine temps will revolve around that basic figure. Let's face it, a 5, 10, or even 20 degree change in the nominal underhood temperature will make very little difference in how quickly your motor heats up, and absolutely no difference in the normal reading on your temp guage, as the thermostat still opens up at 195 degrees.
The claim for ceramic coated headers is to lower underhood temperatures...not your actual block/engine temps. There are several benefits for doing this, but what are probably the biggest two are:
1.) In tight engine compartments, these headers may help protect surrounding components (wires, hoses, electronics, etc.).
2.) As most stock air intake systems draw in air from the engine compartment, any change in the underhood temps will carry some form of impact in performance and, possibly, fuel economy. This is due to the pure & simple fact that cooler air is denser air, and the more air you can get into each cylinder during the intake cycle, the more fuel can be carried in with it.