I'm not an SC owner, and I don't have to conform to smog regulations, but I believe the problem with the cat was not the actual numbers (which a well-tuned 911 should be able to pass without a cat) but the visual portion of the test, where the presence of all the smog-related components has to be confirmed by the tester.
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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net