Confirming approach for P0420

I have a 2006 Mustang GT and as of late it’s been throwing codes (P0420 and P0430 - catalyst efficiency below threshold both bank 1 and 2). I had it into the dealer (since it was initially hard to cold start and would produce smoke), the dealer said it was due to my AFE Stage 2 intake and oil filter which was over oiled and as such some of the oil had transferred onto the MAF sensor. They cleaned it and I had the same problem again, this time I purchased the AFE dry filter, sensor cleaner & throttle body cleaner and went to it on my own and cleaned everything out before reinstalling the...

I keep getting the P0420 code and each time I reset the ECU it comes back after 500 miles of driving. I realize the code is telling me the bank 1 catalyst is below efficiency, but I don't have cats and during my dyno tune my tuner shut off the rear O2 sensors.

What else could be causing this to happen? I recently replaced the IAC motor, I cleaned the MAF, I've checked for vacuum leaks and haven't found anything, there are no exhaust leaks, and the PCV valve was replaced last week (though not sure if it would have anything to do with this).

Now there is another issue here. My car won't...