If nobody has every done the air filter or it is in terrible condition that'll cost you a decent amount of HP. I'd also suggest doing a full intake service, plugs etc. Tho if something were seriously wrong with it you'd get codes so most likely nothing horrible (hopefully). Would run some fuel system cleaner through it after you've done the intake service. Yes it has more power than 2003... also make sure the brakes are in good shape and nothing is dragging, also the condition of the tires/psi, etc can all factor into it being sluggish. Let us know what you find.