K&N Air Filter, Crane Cams XR3000 ignition and PS30 coils, Magnacor ignition cables.
Actually, these were not added with improved performance in mind; they were added because they cost the same or less than the stock parts and offer greater reliability. (they might give a little extra performance but not enough that I can tell).
Oh, and it has aftermarket rims that look pretty cool but I didn't buy them, they came with the car.