Hi Lou,
Money well spent for the long tube headders,answer to your question you do not have to change the cats or the h pipe BUT-- if you are going to spend the cash for the headders than go the rest of the way with high flow cats X pipe and your choise of exhaust my preference is Corsa.when all is said and done you will be happy with the performance and the great V8 sound.
One thing to expect with the mods is getting a performance tune to nock out any check engine light and a crisper throttle responce.
Good luck hope to meet at the Ecklers show.
Mike