AFR idle lower than 14.7

SonicVQ

Overall, it really isn't anything to worry about.

I suspect the cause is your test pipes.

Starting in about 2011, a "Rear Fuel Trim Monitor" was implemented in the ECM software.

As the front Air/Fuel sensor ages and gets exposed to contaminants, it can develop a rich or lean bias.

The rear fuel trim monitor system is designed to compensate for any of these bias shifts using the post cat O2 sensor. This system looks for any bias shifts at the stoichiometric point of the front air/fuel sensor and compares it to the rear o2 sensor.

Since you don't have a catalytic converter and the rear o2 sensor is generally out of the main exhaust steam to prevent P0420/P0430 (catalytic system below threshold) this might provide the ECM with information that it interprets as the air/fuel sensor has a lean bias, so it adds a little fuel.

If it bothers you, I'm sure your tuner could adjust the AFR targets for idle.

Question:
What are your long & short term fuel trims for both banks at a hot idle?