Don't think powder coating will hold up to the temps. There are high temp
coatings for exhaust that would extend the life and preserve the appearance;
check the hot rod magazines for the latest. Have had personal experience with
aluminizing which, although susceptible to scratching, held up well and
seemed to lower head temps. Some of the other coatings may actually insulate
and raise head temps which may or may not be what you want. Wrapping the
exhaust with insulating asbestos or whatever they use now to increase exhaust
temps for turbo applications greatly reduces the life of the exhaust pipe
(but really helps performance). Robert Lilly who built some improvements into
his WBX engine highly recommended some internal coatings but I don't recall
if he did the exhaust. Maybe he will respond.

Ken Hunter
82 Westy 1.9D
84 Jetta w/motor out
89 Mitsubishi van
79 Rabbit 1.5D (donor)