Acually, if you are looking for performance, you may want to consider just buying a muffler and have custom pipe ran with no resonator (I know that everyone is getting sick of hearing this from me). You will get more power gains and spend less money. If you still decide to get the cat-back, madtown is right. THERMAL.
Instead of the AEM look into Iceman...They seem to have more power gains (despite what everyone says).