To powder coat a part, it has to be blasted first to clean/remove other paints/coatings. The part then has a negative electric charge put on it, the powder particles are positively charged, so that when the particles are sprayed, they are 'attracted' to the part. Once coated that way, they are baked in an oven to 'melt' the powder, resulting in a tough, good looking finish. However, powder coating, like regular paints, is only good up to 300-400F at best. So not only would the ceramic coating have to be blasted off, the powder coating would burn right off the headers. Ceramic coatings are far and away the best/toughest coating for high temp parts like headers. You won't get anything any better than that.