I drilled ! started off with 4 10mm holes, no difference in `A` performance or `B` sound, drilled 8 holes, slight difference in `B`, drilled more holes (12 total) slightly louder again, made all holes about 12mm, more slight increase in volume no difference in performance, nothing left to drill now, so next step is to mutilate another standard system (which is knackered) and fit a tiger cub silencer and see how it goes, i did give it a fair try over 400 miles including a trip to holland and back