Headers actually do something unlike a 4-banger header. Header alone doesn't add much, only like 5-7HP. Honda designed the exhaust manifolds to be very efficient. Anyways, I/H/E 15-20HP, not a lot HP for your $$$. IMO the best HP gain for your $$$ is head work(throttle body, intake manifold, & cams) are very helpful. After that I/H/E would be definately worth it.
http://www.mmrusa.com/gudehead.htm
You might find the following webiste interesting & helpful.
http://www.importreview.com/performance.html