Theres no a lot u can do to an SL.. but a good start would be a CAI ... exhuast would be more than 500.. mine was around 300 but i just cut off the muffler and ran back duals and is sounds great..underdrive pulleys would add a little hp, but not a lot... a down pipe would get u around 15 hp i believe.. dang i cant think right now! ..thats some performance mods... check out these sites..
http://www.3sxperformance.com/
http://www.3kgtmotorsports.com/
http://xpp-racing.com/catalog/default.php