I Just Bought A 2003 312 Fastech With 496's. Boat Is Great But I Was Curious If I Could Get More Top End Out Of It With A Prop Change Or Any Other Minor Modification.

CMI headers. Minor, but expensive modification. You may or may not be able to pull bigger props after the change.

If they are the HO 496's you'll see about 77-78 on the speedo and about 72-73 on GPS. You should be spinning 30P 4 blade bravos through 1.65 ratio B1's. If the props aren't right then you could start there. Minor modifications will get you nothing if the setup is already correct.

The CMI header swap is supposed to be good for 45-55 HP on those motors which would get you 2-4 MPH. For $3,500 per motor I wouldn't call that "minor" expense wise.

http://www.raylarengine.com/

CMI's all the way. You will get at leats 4 mph. You will need a prop change if not already at rev limiter. Have first hand experience with that set-up.