I can't tell you about the difference between stock airbox to the SLP CAI. However I can let you know about going from an SLP CAI to a Thrasher CAI. When I purchaced my GTP it already had a SLP CAI and a 3.4" pulley. I bought a Thrasher CAI because I liked the way it looked and I was plesently suprised with a power increase and more of that sweet sweet blower whine.