Yes I have done it and it still sounds good; used a metal punch to each blocked pipe to center a hole to be drilled; then proceeded to a 1/2" hole in each by starting with a smaller bit first of 3/8". Simple modification that sounds good. Notice pipe to left has another plate behind as does the one to the right, after first hole is drilled proceed to the baffle plate which you see in the picture and drill it to using 1/2 inch bit and first hole drilled to guide bit into the baffle so they line up nicely. I would not go any bigger on the holes because your back pressure will be reduced to much and they will not sound nice either. I had read in other Goldwing forums and this was the most successful size hole drilled by others who wanted a better sounding exhaust without having to shell out a lot of dough for pipes. One of our forum members by the name SuperJim had looked at these mufflers when he had them off and told me what he had seen so I could make a good decision before the operation. Good luck to you; I liked the result.
I drilled out all holes on both sides with an 18" drill bit. The last one on the right side was the hardest for some reason. I went through two drill bits about $30.00 total. The sound is a little raspy as others have stated but sounds good at idle. No popping or backfire. I cant hear any difference when riding though. I couldn't wait for torque loops and pipes. Looks like it will not be long now and we will have three options- Torq Master, Two Bros and Delkevic.