The stock exhaust manifolds are cast iron, so there is no special need to coat them. Some have just for the bling factor. I coat my headers to keep things close by from melting (wiring etc. wrapped for extra protection) and for the finished look. I have never been into the wrapped look on headers. Its ok for a while, but they usually looks like hell in the long run. As far as improving scavenging, I will let an expert respond to the value of coating or wrapping headers.