@xcess2 if u want 2 boost the hp i think u should go for cold air intake, if u want 2 go further get a full catback exhuast system;not just the muffler.
1.With the intake your car will be fed with rich, cold and dense air(oxygen) allowing better combustion in the engine hence adding to the car's performance.
2.getting the catback exhaust will allow the car's waste gases to pass out freely coupled with this u will also reduce the back pressure in the valves.