not much on the butt dyno, maybe 5-10whp if you're lucky. You might lose some of the bottom end due to scavenging, but the top end will feel a lot better. You may also note a marginal decrease in fuel consumption, as the header will let the motor breathe better. The cold start emissions will increase a tad, but most of my toys are catless so, personally, I'm not terribly concerned with the cold start bits.