Go 3 forsure. I did mine last week with a Manga High Flow. No diff in my mileage but it seems more powerful in the high end and sounds better even with stock mufflers. Beware tho, I have had to clear a code since due to the cat, so u may need a O2 spoof or tune that deletes the 0420 code.
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K&N CAI, custom 3" Dp, U bend delete, resinator removed. high flow cat, 3.5" pulley, incar Overkill PCM tune, 180* tstat.