All you have is a 2003 GT w/ a catback? How bout some more info? By that i mean, what are your goals? Do you like to drag race? autocross? or just like to drive your car around? How much are you willing to spend?
I would say the KB is a great blower and you should be the 9 PSI with the intercooler. The more power you have the more you want. From what i have seen people try and go the cheap route at first, then they realize that it wasnt enough for them so they wind up spendind more money later. Just my personal thought
-Mike