Hi,
There are lots of Header coatings on the Market, most famous is probably Jet-Hot. Many of these are aesthetic and that's why lots of people use them. But some are functional.
I had the Cast Iron Header on my Lotus Esprit ceramic coated specifically to reduce Engine Bay Temps. I used a company called Hi-Performance Coatings (
www.hpcoatings.com) and used their HiPerCoatExtreme coating. This reduced the skin temp of the Header by 50°F and the Engine Bay Temps by 37°F (though I also did the Turbine Housing on the Turbo for the same reason). Their work was excellent, decent price, good warranty, and fast turnaround, I would recommend them.
They claim a slight HP increase due to greater EGT and increased Scavenge effect. I did several other mods at the same time, so I didn't have a proper baseline with which to measure this. My mods increased HP by 38HP in total, how much, if any, was due to the ceramic I cannot say, but maybe 1-2HP tops. Hope this helps...
Happy Motoring!... Jim'99