You don't powder coat exhaust parts. You get them jet coated or ceramic coated. Powder coating is usually applied in only a 400 degree process. It depends on the coating, but exhaust parts can reach as high as 1600°F. At least one of my exhausts can.

Aluminized steel will rust. Especially if/where it gets scratched, but it's a hell of a lot better than untreated stock. Once it starts rusting anywhere, the coating is worthless.