what size pipe

my car has factory cat to axle pipe, and im about to change it but i dont know what sise to go to, i was thinking 2.25 or 2.5 which one whould be best . my set up right now is b18b motor S-R-I, hollowed cat,chiped ecu and fatory every thing. my 1/4 mile is 15.4 and i want to run 15.0 or high 14 for now whats to quickest way to go . i had 2.5 piping b4 but now more