Actually I was being conservative. Its probably more. A stock GTP runs about a 14.8 quater? Is that fair?
Matt of IPS (developer of the CSC for the GTs) ran a 13.2 with just rockers and a catback. The catback will maybe get you a 10th (thats generous) and the rockers about 3 tenths (again probably generous). That would put a GT with just a CSC at a 13.7 over 1 second faster then the stock GTP.
Now add those rockers, 3" downpipe, catback, fuel pump, injectors, and the IC'd CSC and you have a mid-low 12 second car easily... if you can keep the tranny in one peice.