Full exhaust federally legal if factory cat is used?

I know states have their own laws I just interested in federal law. 2 questions.

1) could I in theory change out an entire exhaust system header to muffler if I kept the factory catalytic converters? I know this is a little dumb but would it be federally legal? Even on a new vehicle?

2) what about using aftermarket cats? Still legal?

 

^
1) Yes
2) If it doesn't trigger a CEL, yes

All hypothetical of course. If your state doesn't have emission inspections, then you could delete the cats altogether and there'd be no enforcement mechanism to catch you. A law that isn't enforced isn't worth the paper it's written on.

 

Or even how well they do the inspections. I haven't had cats on my Mustang for 20 years (do have o2 sims so no MIL) and it passes every time. They plug the computer in, it sees no codes and good go. Despite the car smelling like a lawn mower when it pulls into the bay.

 

^Speak for yourself. My Lawn Boy with Honda SOHC engine (with timing belt) doesn't smell any worse than a regular car. I just put Mobil1 0W30 in it and change the spark plug before each winter