Unless your existing catalytic converter is completely clogged, power gains will be nominal at best. Will change the note of your exhaust; more drone and raspier. Illegal in most juridictions for use on roads and if you have or will have emissions testing where you live, you'll fail.
If you suspect the cat is clogged, at most I'd go with a high flow Catco or Random Technology cat. Otherwise, I'd leave the OEM cat on.
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