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you can do both the slip ons and an air cleaner if you choose. your ecm along with your 02 sensors are designed to compensate for minor mods and they do a great job of compensating for those minor mods. if you go any deeper though get ready to shell out some more money for a sepst or pcv, etc.
you should be good to go with the slip on's and A/C, i ask Jamie at fuel moto about this and he said as long as the stock header stayed on with the CAT. your ok, but if you put a new header pipe on you'll need the sepst or pcv.