If you have a few parts related to each other that are all not in great shape and usually the more expensive ones without FSM saying when to replace in some cases, you will see that on the dyno for sure.
But if your JDM header is doing better then stated, sure, why not. So it must be something is really good on the intake and exhaust and working together.
I was told that Nissan can use a margin for rated HP and you usually can go higher/better bone stock. That's why I'm getting my car up to spec (besides a muffler) and dyno it, or dyno it with another intake. Then I start looking for a nice exhaust upgrade.
I'm messing with it this way for years now, had 1 dyno in my life and the rest is measured by my senses. Btw, I never had a good header under my car