Gaining HP without getting into motor???

Ive got a '99 vette, and am trying to get more horsepower.  Not looking for magic, but have been told that a good tune up, generally gets better results, than most other quick add ons.....Is this usually the case.  I dont want to get into the motor or do anything radical, an extra 15 to 20 HP would be great.  Ive already got a cold air intake, high end filters, and a Corsa catback exhaust on it.....Is there anything left to do W/O getting into motor???   Ive got a dyno sheet showing 403 HP before the Corsa exhaust.....What kind of HP gain did the exhaust give????   Thanks for any help you may be able to give...