I was once in the equipment end of this business, and while I don't know any specifics on this potential application, here are a few things to think about. The powder paints are usually an epoxy, an acrylic, or a blend. They are organics and will burn off at exhaust gas temperatures but in the water passages (where corrosion starts) the temperatures would be fine for long life. The powders are charged and stick to the iron though electrostatic attraction. The iron part is then heated in an oven to melt the powder which, as a liquid, flows out to smoothly cover the surface. The liquid then cures (cross links) to form a permanent, non-melting solid coat. Sounds good but it is very difficult to get a good coating of the powder in interior passages due to the Faraday cage effect. I doubt that complete coverage in the curved passageway of a riser is possible in a typical commercial powder coating setup. A gun that uses frictional charging (instead of a high voltage power supply) helps a lot but can only take you so far. Also after the powder melts (and loses it's charge) but before it cures, it may tend to flow off of high peaks on the surface (such as is the case with sand castings), leading to pinholes. Finally, while the paint is durable, the bond between the paint and the metal is dependent upon the cleanliness of the surface before painting. This would be hard to insure in an internal passage way. Most powder paint failures are seen months or even years after application and can usually be traced back to cleanliness issues.
In short, while I think powder coating would likely help extend the life (perhaps considerably), I would not trust it unless the process for doing so was carefully developed, tested and controlled. And since elbows must be changed before they start to leak, and you really cannot inspect them, it would be a guestimate as to how much longer you would be willing to go before change out.
