I can completely attribute my failed smog not to a cat, but to a failed airbox. I measured with a cat pipe, and it failed miserably, then with a cat, and it still failed miserably and didn't pass until the airbox was replaced. My bet is, you can go with a cat pipe, and if everything else is in order, you'll likely pass.
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Kurt B
'84 Carrera Cab.
carrera_cabriolet@yahoo.com