Removing the primary cat throws a P0420 CEL, and because NBs are OBD-II (1996+) they do not qualify for rolling antique/classic exemptions in states that test cars under 25 years old, so smog compliance is state-dependent.
Emissions: cat delete P0420 and the OBD-II 25-year rule
Mazda MX-5 Miata (NB) · ownership