Maybe I missed it, but has anyone just said to unbolt the plumb back section of the ext gate?
Would take about 5 mins at the workshop, run the car up and see what happens. You're bypassing the exhaust then. I bet your car will be happier.
Now, obviously you don't want that for the road if your daily driving it, but it's going to give you an indication.
From my experience, I have a true 3 inch exhaust, a "high flowed" cat, when my gate is plumbed back power drops from 289kw to 272kw with nil change in boost.