keep the stock cat in place since you will be illegal and will not pass cali emissions.

if you want add either a second resonator in or a longer resonator than what you have. most exhuast shops only put in a 14-16 inch resonator, but you can fit 2 14 inch resonators or one 26-28 inch resonator no problem in the spot after the cat.

since your OBDII and live in cali i would keep the stock cat. honestly i wouldn't waste my time with a custom exhaust since cali is so strict about that stuff. i would buy a apexi ws exhaust and shouldn't get harassed by cops unless they are in a bad mood that day.