well i suppose before the settings were to compensate for the s15 injectors but since now your on the stock ones again i think what you should do is get a wideband and check what your air/fuel ratio is at throughout the rpm band. so you can accurately adjust the safc.
hm is the back fire between shifts? do you have a venting blow off valve? that could cause that....
what i think you should do is go back and start from the beginning and just go through everything.
maybe a full tuneup, check all your voltages are correct. i wouldnt use s15 injectors unless your ecu was tuned for it and im guessing you on the stock ecu.
kinda sounds like your car is running really rich...