First of all, what is your complete timeslip? 12.7 is a quick ET but what is your 60ft and what is you MPH? Unless you have a limited slip differential and at least drag radials you probably can get your low 12 second pass with just these additions.
Now, once you have that done, consider that 150hp on top of the stock engine ECU programming is already a dangerous choice. The timing map for safe running is likely less than you are getting now. I would strongly suggest getting the car into the hands of a competent tuner that can check A/F ratios and timing under power.
Now, once the above is considered... remember that nitrous is chemical supercharging. The extra power comes from the additional fuel and oxygen (nitrogen also plays a role) and you are effectively putting much more into the engine. It now becomes more important to get it out. I would suggest an upgrade to 2.25" exhaust pipes feeding a quality Y pipe and a 2.5"-3.0" center pipe. The manifolds themselves are a problem but until headers become available you don't have much option other than custom builds. On you nitrous application I believe a full exhaust (headers, stock cats, upgraded balance) would give you a good 50+hp.
As you pointed out if you remove the cats your 2nd O2 sensors will trigger check engine and effect performance. There are companies than make O2 simulators or "sims" for short. The Corvette guys run them frequently. You could probably find a compatible sims for the GS.
.. are you racing vipers or something? just kidding.. those are some very good results..
