Unless you get a tuned muffler setup you will loose something somewhere. That is why i suggest tuned cat-back kit from exhaust manufacturers.
Certain egr codes can be cleared up some others require replacement.
420 & 430 indicate that the cats are wearing out chemically. as long as the substrate does not melt you should be fine performance wise for the most part.
Hmm, I tried to race few other models the results was great I know it is only a muffler not a super charge but still I am getting good results for mid and hi range what I am concerned about is the low range I think if I added the right CAI I could solve it but before that I want to do the throttle body replacement with bigger one, wait this is the list:
1- K&N 57-3013-2 - K&N 57 Series FIPK Air Intakes
Or
Volant 15957 - Volant Cool Air Intake Systems
2- MSD Ignition 8231 - MSD Blaster OEM Replacement Coils
Or
Accel 140024 - ACCEL Remote Super Coils
3- MSD Ignition 32169 - MSD 8.5mm Super Conductor Spark Plug Wire Sets
4- ACDelco 41-993 - ACDelco Professional Platinum Spark Plugs "have it waiting for fitting"
5- Hayden Automotive 679 - Hayden Fluid Coolers (trans cooler)
6- modified throttle body.