Jet Hot coating will keep the exhaust pipes from burning the cowling. They coat inside and out of pipes. The downside (if any) is that exhaust gasses exit the end of the pipes at or very nearly the same temperature they exit the exhaust valve! Be careful where they point!
I have a set of Hal Hunt pipes for O-200 that I had coated by them, and also a 6" Sabre prop extension not yet installed. My concern is the heat of the exhaust plume aimed at prop disc. If the prop is "clocked" correctly the hot exhaust exits between blades, but I still plan some very short runs followed by shutdown to see where prop temps are going..
A set of those pipes are up for sale on the forum right now.
BTW, I have used Jet Hot on race cars and motorcycles for years. my experimenting with other less expensive coatings did not turn out as well.
To put the snake oil guys at bay, I assure you that when I coated race car headers, a carburetor jet change was required to take advantage of new found power. when I treated IO-470C exhaust manifolds and down pipes on a Bonanza, there was an immediate gain in static RPM and takeoff RPM on takeoff. That in itself is an obvious indication of power increase. I got the same boost that seaplane guys have been doing for years when they "adjust" the prop governor for more takeoff RPM. Also, after race car shut down, and immediately after exiting the Bonanza, I was able to place my hand on the exhaust header and the Bonanza down pipe without leaving skin on the metal ...palm, not finger.
When I did the Bonanza, I conferred with a DAR and an FAA rep who both said that is was akin to painting exhaust and not considered a major change. I'm sure some would disagree, but common sense prevailed then.