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...  

The shop that Dyno'd your Vette should be capable of a "re-tune".  I'd be surprised if they didn't offer to do so when you took it in.  I'm assuming this is not a Z06?  The 403 HP dyno HP you indicate is very high for the mods you've listed.  If this is at the rear wheels, you've got MUCH more HP than most any other C5 with similar mods.  A really good set of long tube headers with high flow cats and a good set of mufflers is typically good for about 25 to 30 hp at the wheels when coupled with a free flowing cold air intake like a Blackwing or Vararam.  If you want to move up again, you'll need to go to the cam and perhaps an underdrive crank pulley.  Depending upon what you put in (mandatory retune at that point) you can gain another 25 to 40 hp at the wheels.

Good luck..