420 and 430 are just the codes for
the rear cats. You just added the one code by cutting off the one good cat. The problem you may run into is the fuel trims need to be re-cal'd for the extra air flow. Your fuel mileage could actually go down without tuning for the extra flow. You need to get it tuned to turn off the codes and adjust the fuel trims. You'll be good to go after that. You can gain 30-40HP, get your shifts firmed up, get rid of all that darn torque management plus fix all these issues at the same time for one cost.
I'm new to this forum but have had a tuning business and have been tuning for about 5 years. I can help you out or refer you to someone more well known if you'd rather, since I'm so new here. I just bought a 2000 Tahoe and want to tap into everyone's knowledge base to mod mine.
