Catalytic converters

I thought they might make a difference. I used to occasionally leave our cruise nights last just to watch the cars all leave.Problem was you ended up with stinging eyes and sinuses from all the idling from the fumes from all the cars.I'll bet a few well tuned engines burn reasonably clean but it's awfully difficult to clean up the idle on big power builds , or engines with long duration cams.
...wonder how much performance loss there would be with dual cats?