I wouldn't expect that much power from just headers. You will probably see an average of 1.4hp. One of the magazines did a header test, and the one that scored the highest was a 4-1 race header (~+10hp) that just dumped under the car, no cat or muffler.
Just slapping on one part like that can't yield much power, because there are other bottlenecks in your system. Sure, the air can get through header fine, but what about before and after that?