Good question. We have strict emission testing here on Long Island. The car must go on a dyno. They don't really do much of a visual ala California but we must pass the smog test. The aftermarket cats I have now do pass. I have to make sure the wrench that inspects my car annually would agree to helping me out if it were close, if you know what I mean.
Dammit. I'd also like to save for cams, valve springs, a tubular K-member and a .........