Coating Manifolds: Up until now all upgrades I have listed can be done by most anyone. This however is a VERY extensive upgrade and should only be done if you're an absolute power freak or if you have allot of time on your hands. This upgrade involves removing your stock exhaust manifolds from the engine and having them ceramic coated inside and out. The benefit is that it reduces underhood temperatures by 100-120 degree's. An underhood temp drop means a colder intake charge, which means more horsepower. Reduced engine temp means longer engine life and improved exhaust flow is also attained because of the coating on the inside of the manifold. It cost's $145.00 and you have to send them to Performance Coatings Inc. to have it done. Information about there service's can be found at http://www.performancecoatings.com.
Ported Manifolds: Ram Air Technology offers ported stock manifolds for improved exhaust flow. These run $239.00 and offer a 5-6HP gain.
Manifold replacement: Both Ram Air Technology and T.E.P. offer headers available for this car. The Ram Air Tech headers will work with your stock catalytic convertor but the T.E.P. headers will require a new downpipe be welded to your cat. Price and dyno numbers are unavailable for the R.A.T. headers but T.E.P.'s headers are $865.00 and offer a 18HP gain with no other modifications on a stock car. I have seen and heard these headers in person and I like them. They even make a stock whisper quiet GTP exhaust system a little loud, its not recommended for a already loud exhaust system like the R.A.T. one.