Now were talking. There are a lot of free mods you can do as well. The less weight in the car, the better off you will be as well. Here is something to think about. If you have full coverage, and a towing option, then there is no reason to carry a spare, or jack etc. Look at lighter wheels/tires as well, the less rotational mass you need to turn, the more power you have in the engine. A smaller battery, like a Hawker Genesis can drop 10 lbs in itself.

Cat backs are exactly that, from the cat back, they will change the tone, but yes you can free up a few hp with them. The problem you get into when removing a cat on an OBD II compliant vehicle is that the computer will know it is missing, and set the check engine light unless you put in a fooling system. This is a unit that plugs into where the secondary O2 sensor is located and sends a false signal to the computer to fool it into thinking that it is working. Removing a Cat is a federal offense, so do not expect any muffler shops to be willing to do this for you.

Be carefull with some of the aftermarket air filters, most are covered in an oil, and with a MAF sensor, they can cause some problems with the fine wires in there and burn them out.

Read more about it here: K&N air intake - RX8Club.com