Sir: You have a few things going on there and a few suggestions/things to check as follows:
1-A "backfire" on decel thru the muffler is a fuel or air leak related After-Fire, caused by unspent fuel accumulating down-stream of the exhaust port and igniting inside the exhaust system if a few parameters are reached.
2-Most stock XB9's do NOT pop on decel....Most stock XB12's do NOT pop on decel until the IEV is either over-ridden OR wired-open OR removed.
3-Your ECM is trying to enrichen/add fuel by changing both the injector timing as well as opening duration. Something askew.
4-There have been 3 posts on here in the past 6 weeks of XB owners with satisfactorily running models, who then added a Revision-Moto generic or "recommended tune" who subsequently had both throttle response and overall performance issues with same.
5-Highly unlikely that your O2 sensor is at fault. If you've received either historical fault codes 13 thru 15...it typically is not the related sensor...it is a failing ground. Check all of yours INCLUDING ECM grounds.
6-Carefully check every clamp...connection...mounting point....hanger...flange...for your ENTIRE exhaust system. A crack or leak anywhere on same will result in significant "decel after-firing".
7-If you'd like, I have several ECMSPY CD's and diagnostics cable that have proven generic race maps installed on same that work quite well. Have installed same on probably 25 assorted XB's and gives a nice improvement over stock mapping. I can lend you everything if you'd like and perhaps this generic map would solve your problem?
Let me know via PM if that would help.