I don't expect that barrel harmonics can move POI by 15 cm at 10 metres - that is a huge change!. It must be the mod either clipping or not quite centred on that rifle. You could try to catch a pellet in something very forgiving and see if it has clipped. Also try fitting a washer between the barrel and the mod - those fibre plumbing washers come in 1/2" and should be a good fit. If still strugling then try adding about the same weight to the end of the barrel (without the mod) and see what effect that has.
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