hmm I don't believe its going to hurt anything. I see in your sig that your 3 turns out on A/F screw. Try some 42 pilots jets. Also by adding shims this should correct your backfiring when letting off the throttle. Are you using #4 brash washers for the shims? If not try adding one more shim under each needle.
Can't say that I heard any backfiring myself. I heard a little blurbly sound when you let off the throttle after running it hard. My setup with the HMF duals does the same thing. I think it is normal. Someone else will have to chime in and verify that because I am not positive on that but I am pretty sure it is normal.