I have ran the GYT-R carbon muffler with the head pipe, and it works awesome. I have seen the dyno charts and the head pipe makes additional power from 7500 to the limiter, through that range it makes upwards of 2HP (progressive).
i'm currently running the GYT-R carbon pipe on my machine. i dynoed it with the GYT header pipe and with the stock header pipe. and for my money i'll keep the stock header on. it makes more torque then the GYT-R header and only loses a couple ponies over 9000 rpm,but thats where the power drops off anyway. so to me the stock header is about as you're going to get. just get a slip on pipe and go from there.
heres a link to a previous post i put on here for my mods and a another link for a pic of the dyno results with those mods. my yfz pics and dyno results thread