Hey Chris
Bad news about the defect.
Here are some answers;
You could stick a baffle plate between the flange, just make it out of 2mm sheet yourself. Otherwise you could get a baffle that bolts into the back of the cannon should, that should get it under 90DB, then go to fj mufflers at cardiff or any good exhuast shop and get a test done.
Passing the IM240 isnt easy and leaner can make it worse. Best off with the stock ecu air filter and a stock cat(used is fine) if you can get it modded to fit on your exhaust and then go for the test and see how you go, failing that you might need a full stock exhaust and/or injectors cleaned.
Rta also have a testing centre at botany, they do test there on wednesdays, you might get in there earlier.
Let us know how you go mate
Josh