Ive had my car for quite some time and after adding a CAI a while back I fell in love with it. As some of you may know I just recently replaced my lower intake manifold gaskets and cleaned everything like crazy. Today after sorting out some problems after reassembly I took it for a test drive. I noticed a really nice gain in performance and "pull" when I pushed the gas and I couldnt be any happier with the results. Ive had the thought of getting a Magnaflow High Flow Cat from eBay and a Full Boar Muffler to make a dual exhaust setup. I just want to know what kind of gains I can expect as well as how it will sound and feel. How much different did it feel for some of you guys that have an aftermarket exhaust? Im very excited about this and cant wait.

You might get 5 HP from each. The muffler is mainly for sound. But I do have a high flow cat with my larger downpipe, and I did notice a nice difference with it.

If it has a supercharger you could gain 10 hp and the ability to increase airflow with a smaller pulley.
If not you will lose low end torque if not careful and choose your sizes correctly.

As long as you don't upsize the pipe you're fine. Consider adding a resonator, it quiets and smooths the tone.