Karl these sites have a few ideas, I'm not a M3W owner so don't know the details but this gives you some idea of what is available until one of the guys that has one comes along.

https://www.demon-tweeks.co.uk/performan...g-sound-barrier

https://de.merlinmotorsport.co.uk/knowle...lin-muffler-270

2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev