I was wondering what the hp gain would be from removing the cats in place of an off road x-pipe only, with no other mods in the exhaust. I have heard about 20 rwhp from long tubes alone, but no numbers on the cat removal only.
I believe that without a software modification, your performance would get worse as the computer will not sense o2 correctly with the main cats removed. Even if you did the necessary software changes, I doubt you would get much out of it.
I wouldn't mess with the main cats, just get rid of the pup cats.
The computer has no problem adjusting to the minor air/fuel change, the is due to the less restrictive nature of not having cats. O2 simulators on the 2nd set take care of the computer. I installed a full off road/ x pipe, all cats removed, the car will be dynoed in the next couple weeks to document as the only change. I'll post the results. I'm suspecting about 13 hp or so. The biggest plus is the sound, the stock Ti's sound Great now!!