Cat back exhaust
I just did the reverse, I kept the stock cats and installed a Rogue Engineering DMS 36 catback exhaust. I wanted to keep the DB down since I still drive on the street occasionally. The exhaust helped significantly at higher RPM's the dyno indicate 5-7 HP on consecutive runs.
Then I installed a chip (ebay clone), moves the redline to 7200 and increases fuel flow at top end, plus removes the top speed limiter. This change gave me another 12-15 hp at top end on the dyno. So the pair gave me an honest 20hp at the wheels.
I personally believe the muffler represent far more restriction on the exhaust than the cats, so I did this first.