Did you have a full exhaust put on as well? When I first installed my headers I had them hooked up to my factory exhaust and the results where less than exciting. A week or so after installation I decided to go down and buy a new cat and a flowmaster along with enough 2.5"pipe to make up a new exhaust. Once I finished the exhaust the performance improvment was very clear.

I didnt use the bolts that came with the downey headers I opted to use studs. I actually used the studs that came from my old exhaust parts. there are 9 longer studs that are used to hold the factory xover together and I bought 3 more from the dealer and use those. I also cut up a pair of factor manifold gaskets (the sheet metal ones) and use those instead of header gaskets and thay seem to work ok.