how much hp gain with header

You've got carbed, but I don't see a reason why it can't offer you gains simular to what I got. I don't see being injected as a reason for it to gain more from a header. Just my 2 cents.

I felt that with the gains I got from adding a header, were totally justfied in it's purchase price, worth every penny. And I paid $300 for mine. I highly recommend that anyone thinking about one get it. It made good gains on my car and should on anybodies else's car. I didn't loose any lowend, gained midrange and high end, with power now to redline, instead of dying short at around 5700rpm's.