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So apparently there are NO emission laws in any state that doesn't allow you to add..

BATMANs

BATMANs

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OBD-2 aftermarket cats after the the OEM cats (in the OEM location).

In other words if ur in California (where most NSX resides in the world) and don't want to chance ur car failing (or worse - flagged as a gross polluter) the sniffer test it may be a viable option to have a custom CB made so that there are the largest OBD-2 cats that you can fit in place of the mufflers.

Emission laws don't govern mufflers/sound.

I know this from personal experience.

Discuss......
 
I am not sure how much that is going to help. If its a conventional 3 way cat, the cat surface needs to be periodically recharged with O2 for it to complete the chemical reactions associated with combustion clean up. That is why when you look at the short term fuel trims, you will usually see them bouncing back and forth going from excess to insufficient O2 by very small values. Whether the OEMs design that oscillation in or its a happy un designed by-product of the control loop, it helps to facilitate the clean up. If you add a set of cats after the OEM cats, they are probably not going to get that O2 so the second set of cats may not be very effective. It probably won't hurt your emission performance; but, has no up-side unless your OEM cats are totally dead and not capturing that O2.
 
Is your Mazda a direct injection engine? Some of those direct injection engines implement some exotic combustion strategies (switching between homogenous and stratified depending on operating conditions). DI engines also typically run with much higher compression ratios which means that the stuff coming out the exhaust port is different than what it is on the C30 / C32 which are late 1980's technology.

If the Mazda is conventional injection, Mazda might have decided to split up the converter duties with one handling NO and the other two doing CO and HC. You would have to figure out what Mazda (and others) might be doing with their cat arrangements. That might provide some guidance on the 'would a second cat help' question.

As an observation, the catalysts in the cat don't really wear out. They can become contaminated. Metallic additives in the gas (lead being the obvious one) will do a cat in quickly; but, unless yu have been fueling up with race fuel that should not be a problem. Cars with high oil consumption or poor fuel mix control will contaminate the cat surface or cause an overtemperature in the cat and a bought of engine misfires will also bugger the catalyst up. Other than that, it probably death by rust holes.
 
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