Chris, I am not sure by your answer that you are completely clear on cat removal. You should also state year of car that you are talking about. My car is an '02, 2.7 and has four cats, the first two are built into the header and have O2 sensors fore and aft. These are the ones that determine if you pass smog or not. The second set can be removed without affecting the smog test but violate it visually. I put a set of 2nd cat delete pipes on for less weight, less heat and more sound. A bit more power is claimed but not enough to get too excited about. So you can remove the set without the sensors with no problem but will fail smog if you mess with the primary header set. Others on this list have experience with putting on aftermarket headers and such and will have to chime in with further advice.
Hope this helps, AKL
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