I personally like the SHAZAM button on these motors. For the most part, they make the most power with higher timing. LS motors and many others, that's not really the case.

Adding timing may affect idle somewhat. You can play with it and see where it idles the best.

As long as you are still running and O2 sensor, you can't really do anything to change your O2 levels. The only thing you can really do is dial in your fueling so your LTFT's are within +5 or -5.