450 is a lot more out of the stock charger, then us cummins guys get. Which means it flows easily well enough for a 650hp towing setup.
If you are set on replacing the stock charger, I would look at: (in no particular order.) GT35, GT37, BW S360, BW S362, Precision 62mm billet. BW's have become way, way, way over priced, companies like II, and HTT, ect... Have been changing the turbine housings, and then been overcharging till it has become the norm, to pay $1500+ for a $500 turbo and a $200 turbine housing.
60mm class obviously lights a little earlier, and the 62mm class will take you into the 700hp range if you so decide.