installing the stock full exhaust will just make it quiet and really slow. No tuning needed really, as your mass airflow sensor will tell the ECU how much air is being flowed.
now, that said, I agree with Jason, modern good quality performance cats are really good. Not some cheapy one, but a good one from Magnaflow or Walker. Walker also makes a new one called a Calcat. Those are really good! I recently did an aircare on this old Cavalier((or Escort??) Either way a 25 year old POS domestic that could hardly run, had no power, stunk, was just a regular lump of crap in general. It failed MISERABLY. Like 10+ times the amounts on ALL its readings. We did NOTHING else but put in a Walker Calcat and if FLEW thru!! Less than average readings on all, some were half the average readings. If the customer would have let us repair the car properly, he could have gotten away with a cheaper cat, but what I am saying is those cats kick ass.