Just put a supercharger on my lq9 and got it tuned Sunday and it ran fine on Monday I put tr6 plugs ran fine Tuesday and today it starts to backfire at low rpms and when you press the gas it continues but at wot it runs no problem. No engine codes.
Set up Is a lq9 with l92 heads,cam,42# injectors and walbro 255 pump

 

Symptoms point to a valve not sealing until combustion pressure forces it closed. A leakdown test would disprove my theory.