If I remember correctly the shorter pipe increases bottom end and the longer pipe top end. The diameter of the pipe plays a role in this as well but Im not sure how much compared to the pipe length. The stinger opening plays a small part in the power curve as well as with the sound of the pipe.
If you arent getting enough acceleration off the line I would go with heavier clutch springs. You probably have .9's in there now. Maybe try 1.0 and see if that improves the take off. The heavier spring will cause the engine RPM's to be higher before the clutch engages so it will take off quicker.
Any pictures of the damage?