hi, i dont know if it helps much
but i have an Atomik 125 and when im tuning it and riding around home, i just remove the rear muffler and put a washer with a 13mm hole in it with the same od as the pipe and slip the muffler back over it and do up the clamp. i cant really notice much of a power loss, if any.
it quietens the exhaust down about 25%, my muffler is a modified china one.
the neighbours havent complained since....and the price 10c or so from bunnings, cant beat that!
when we go out riding i simply remove the washer and it's back to being loud again.
another idea, a guy on ebay from melbourne? sells a muffler tip insert which has been CNC machined to fit and comes in a few different sizes too.
he reckons it quietens the exhaust around 20 decibals and actually gains some bottom end torque too
he sells them for $25, you just slip it in the tip and put a self tapper screw though the new tip and your existing pipe. easy enough to remove it when you are out in the paddocks or track etc
i tried looking for him on ebay a month ago but he hasnt been selling them lately? or i have been looking in the wrong place