K&N sells a drop-in panel filter replacement for these cars, but it doesn't really help performance much. I've got it in my car simply for the maintenance aspect. They say that you only have to take it out to clean it every 50,000 miles (I'll at least visually check mine more frequently). This may sound like a trivial improvement, but the air filter on these cars is a PITA to get to! Wetterauer sells a similar free-flow panel replacement from RamAir that is claimed to provide better filtering.
K&N also sells a cone filter replacement that is more likely to give the car a little extra power. If nothing else, it makes the car sound cool. The picture to the right is a cone filter setup. There are concerns that hot air from the engine bay will counteract any positive benefits that the cone provides, so it's a good idea to build a heat shield. You can also try the cold air intake route, performed by iProDrive.
