Well...I manage a smog test & repair shop here in CA. The high flow cat, like Magnflows (the brand of cats we use) will get you thru smog at least once. The specs are higher on our MKIIIs than newer cars. The high flow cats are just that, and many times the openess does not allow the cat to get hot enough to do its job.
I installed the 2 1/2" cat and piping all the way back. It seems to work fine. I am only 225 hp, though. My smog #s are very good.
Be sure to get a new O2 every few years. Cheap insurance for better fuel mixture ratio.
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