rolla junkie

I have a 89 gts and I just installed a KBD header which I bought off Ebay. Has anybody else ever put one of these in? My car is now much slower through the entire rpm range. It claims to add 6-10 hp at the wheels but exactly the opposite happened. My car is basicly stock except for a cherry bomb resonator which I put on a week after the header and it seemed to help a little bit. If anybody has any ideas please help me.