Thanks Damo, my 550 is at 160,000km. Still runs well, but I think the middle muffler is starting to clog up. From my reading the dump pipe construction is the most difficult. The after market systems separate the Cat from the Dump pipe which maybe a good idea. I also see Dump clearance is difficult. It needs good clearance because of the heat. Good to hear the 3 inch system improves performance. I checked the legal requirements in Victoria, which are very broad. Basically less then 20% power gain, maintain all the original sensors and Cat and its legal.
I think there’s also a bit of a grey area with the DPF in the 2011/2012 models (mine has 130k) as these were basically passive systems that weren’t plumbed in. They are also easily 30kg of cast iron. I think as long as you keep the cat then you’re legit.
if you search kuluin exhaust on YouTube you’ll see the system I picked up but I know there are other ones around.