There is a K and N filter available for your car. This is the first place to start. It's worth about 5 to 10 hp and will also save fuel. It also pays for itself as you never have to change an air filter again. After that their will be an option to change the exhaust (you might have to get one custom made as there are few 400 series cars in North America) but again 5 to 10 hp gain. The rule os about 7% improvement is the maximum you will see from either of these and average of about 10% for both. Ten percent may not seem like much but the car doesn't weigh that much and the throttle response will improve as well. Any other parts will have to come from europe so feel free to search over there. Because the engine is also used in Renaults there should be a chip out there. You can also try changing the spark plugs to Iridium. You may find a difference with this but it isn't huge. Maybe a couple of ponies. Good luck.