Offroad-Express

Putting new O2's will give you the best result (unless you have a plugged cat). If you put exhaust that has a higher flow, you will need to match the intake and swap out the cam for more lift. The cheapest and easiest thing to do is to cut off your rear resonator and turn the pipe down. I would also pull the silencer trumpet from the airbox. It will be a little louder, but it will improve flow.

I would not suggest pulling cats. If you feel like there is too much restriction, put some high flow Magnaflow cats (or similar) on. You will not see much improvement unless you spend a lot of money on airflow..

And if you start throwing codes:
P0420 = Catalyst efficiency below threshold ( Bank 1 ).
P0430 = ditto ( Bank 2 )
P0432 = Main catalyst below threshold ( Bank 2 ) .
There's a whole slew of codes for catalyst dysfunction out there.

Better yet, sell it and get a Nissan

Also you mght find that with new rules( not sure when they are going to be inforced but) if the car has had cc as factory , then you are obliged to keep them for woffs etc.. someone esle may chime in to confirm..

I doubt they'll enforce that pre euro 4 to be honest.
just hack them out, slam a couple of resonators in there and be done with it.