Recent thread by Metal_Head titled "Rear Muffler Delete" included posts about an ideal exhaust system size application for the Aurion. Preliminary research confirmed his suggestion of replacing the last catalytic converter with a 3" version as an ideal starting point for an improved flowing exhaust system.
On the 40 series Aurion, there is a very large resonator after the catalytic converter. Approx, exhaust pipe measurements [after the flex joint for the front Y pipe] increase from 60mm to 72mm so this should readily accommodate fitting a 3" high flow stainless steel catalytic converter