Sorry I can't recommend a good GW wrench in Northen Calif.
First thing I would do is re-check that you have no exhaust leaks between the loops and header pipes and the connectors between the slip-ons and the loops.
Next, do you blip the throttle when you down shift during deceleration. if so STOP IT and try down shifting by not blipping the throttle...see if there is any change.
A lot of more open exhaust like the "Rush 2.25" in conjunction with loops will reveal this normal deceleration popping characteristic on EFI systems that stock mufflers hide.
I run the "DR. Jekill and Mr. Hyde" https://jekillandhyde.com/ adjustable muffler with loops and when in the open mode (straight through) I get deceleration popping, but with mufflers closed I don't hear any popping.
DR. Jekill and Mr. Hyde muffler
DR. Jekill and Mr. Hyde muffler open
DR. Jekill and Mr. Hyde muffler closed