I put on a Supertrapp (slip-on) this weekend. The muffler is 2" diameter and the stock GS pipe is something like 1.5" so I had to do some welding but it came out cool.
At WOT it would pop and there was a noticeable DECREASE in power. I removed the end cap and plates and it's all better. Feels like a BIG increase in low-midrange, lots stronger. Sounds good with the pipe open but it might be a half-click too loud for the local authorities. I am jetted at 150 mains, stock pilots, 3 turns out on the screws, and a 15 tooth front sprocket. It feels like a Yosh-GS that I rode once so I am happy.
When I had the pipe off I noticed a significant ridge around the ID of the exhaust tube where the exhaust exits the head and enters the pipe. I took a die grinder to it and ported it out a bit. Thought I would offer that up as a freebie-hp if you have a die grinder or files. It was obviously an obstruction - probably not worth much but it's free...
I love my GS!!! :cheers: