Generation III External Engine - Jet coating headers question...

How significant are the benefits (if any at all) of jet coating a set of long tubes as opposed to not coating them at all? Wouldn't jet coating actually retain more heat?

Well, first of all its "Jet Hot" coating ;). That is the whole point, it keeps the heat in the exhaust, and cooling down your engine bay temperatures. Aslo if you dont get stainless steel headers, the coating helps prevent rust from developing on the headers. Tho some people complain about the coating peeling off the headers.

My coating is pitted/rusting on the bottom of the car where its exposed to the elements. In the engine bay, the headers are still as shiny as the day I installed them just about.

Jet Hot coats the inside and outside of the headers, what this does is keep the heat in the exhaust pipes (keeps gasses hotter and flowing faster), which prevents the metal of the headers from absorbing as much heat and having to dissapate it. Then b/c they are coated on the outside - engine bay temps stay down. But don't let that fool you, those sons o' bitches are hot as hell right after the cars turned off ;) But they do cool down pretty quickly.

Bottom line:
It's better than not coating them at all, but as good as stainless is probably an overstatement(in terms of durability). But for the money ... its a pretty good deal.