Just completed this mod on my stock gtl exhaust this afternoon. Used a long 1/4 " bit to drill upper and lower tubes centrally. The sound was slightly deeper. While my adrenaline was elevated I then opened them to 1/2". The sound is appreciably deeper and noticeably more throaty. Test riding, I convinced myself that I noticed a little more power but minimal if any by seat of the pants dyno! The sound however is much nicer at all rpm levels. It is not annoying or disturbing to the rider, riding pals or loud enough to disturb others. Drill slowly and keep the bit from over heating by using a penetrating fluid. I removed al metal shavings with my trusty telescoping retrieval magnet . Took approximately 30-40 min total. I am pleased with the results and will certainly report if any negatives develop.