[fast924S]

If I where you, First get the header, I noticed it made a good difference in torque down low and more HP up in the R's, Also if you can get a header rap kit, this will help keep temps down in the engine bay. As for running with out a cat, I am doing that now and yes it is louder but I like it. If you can get a good cat-back exhaust system with a good resenator then you should be ok. Another thing you can do is remove the old cat and punch out the material inside and reinstall This will help reduce some noise because of the big empty chamber. I havent done any Dyno tests but I did not notice any power losses from removing my car. Backpressure would only be a problem if you made the DIA. of the exhaust greater then 2.5 inches. Good luck and let me know how it goes