If your car isn't tuned for that downpipe, you need to get tuned ASAP. That should be your priority. I would recommend not even driving around without at least a stage 2 flash from Cobb.
The fluctuating AFRs at idle have to do with the fueling at cold idle, and the air pump blowing into the exhaust at startup. Worry less about AFRs at idle, and worry more about AFRs under load, boost, and WOT. STAY OUT OF BOOST UNTIL YOU GET TUNED.