Just the one back box removed.So you have no back boxes and one of the two silencers gone?
I have the same car and engine and have removed just the centre silencer, still have the backbox and the other silencer near it.
Not much louder to be honest but didn't want it too loud as I use it for visiting clients as well. The garage have said they can remove the second box if needed but wouldn't remove all 3 due to the drone.