Depends on your budget... A replacement filter such as the K&N (I recommend BMC) will make the car a little more driveable and probably give you about 1kw difference. This will be about $100-$150 depending which you get. A CAI of some sort have been found to make the car more driveable again and will give you approx 5kw difference in total power (Just depends what you get - there's a few options out there now). This are about $250-$450 depending which you get. An Axle back muffler will give you more power (Stay away from cheapo cannons) and can be anywhere from $300-$850. A Cat-back exhaust will give you even more and will be around $650 - $1200 A set of extractors/headers or porting existing headers can set you back approx $300 - $800. I consider all these to be light mods as your not really touching the motor, but it just depends on what you want to spend and what you want from the car. On my Conquest as an example, I just wanted a little more poke and a nice exhaust note, and didn't want to worry about epa or fitment problems etc etc, so I fitted a Remus rear muffler ($850) and a BMC Airfilter ($150) and removed the stock snorkle from the airbox, Nice improvement all round and it pulled 79KW ATW. Compare this to Danz Car who has a full exhaust system and an AEM CAI and his car pulled over 85kw ATW on the same day (But has done over 90kw ATW before) Although it doesn't seem like much of a difference on paper - his car has quite superior torque meaning on the road his car eats mine up. So just decide what you want from the car and what your willing to spend