You cut the cats and the downpipes, and the reprogram from stage1 to stage2/2+. It will not throw a code if the o2 sensors are put back correctly. This is the common problem that your talking about getting piggies, Warm up on Bank1 / Warm up on bank 2. I have not had a light in a long time for piggies

 

If you gut the cats the CEL will be on. That puts your ECU in limp home mode which, as somebody already pointed out, will use more fuel and perhaps make the engine run lousy. Most engines (v6) these days have 4 O2 sensors. 2 are pre-cat, 2 are post-cat. The ones B4 the cats set fuel/air ratio. The ones after the cat are there to make sure the cats are working. They want to see an exhaust stream that has no surplus oxygen.
How much extra HP do you get if you gut the cats?? No worth it, IMHO.

 

so the last two posts are pretty contradictary of each other. can anyone clear the air?

 

OK... spend 200-400 gutting the cats or spend 1600-2.5k for cats and downpipes? Not worth it I think not. I think this is worth it if your cats and downpipes are in shape. You will get 40HP prob with the ECU flash. This is really ment for opening up stage3. You will be holding your car back.