well if your only running 3" after the cat .. i would run 3" before the cat too ...
im running 3" from turbo back, running stainless custom with an xforce muffler..
cost me $1100 including the new cat..
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remember hollowing out the cat with induce flames on the overrun etc, and will potentially melt stuff, as well as be very illegal ... big fines and EPA tests etc.
up to you on that bit. i say dont do it