ok OP i have the same set up as u plus a cam i also get back fire but trust me its perfectly normal for this set up. its because the bullets are straight through, no baffles or anything, and when u shift or decel at just the right rpm the sudden change in flow through these really open free flowing pipes will let out a pop every once in a while. my car is dyno tuned and runs tip top. but if your air/fuel is off a little i'm sure thats not going to help so a tune couldn't hurt but it'll never completely go away with this set up.
^at the end of the vid when i am driving away then decel listen closely...its normal.