How much horsepower can you get from intake header exhaust?

I believe you meant to ask about an EXHAUST header. Most headers advertise a 10-25% hp increase. I believe that a more reasonable figure is 5-8% depending on the quality of the headers. "Tuned" headers tend to yield slightly higher gains because each runner is supposedly the exact same length so there is less backpressure. Some back pressure is okay, so don't try to eliminate it all.

Personally I have fabricated my own exhaust headers for quite sometime now, I don't use same length runners, instead I change the rocker arm ratiOS to adjust for the shorter/longer length the exhaust must travel.