I would give BCS a call they do a good back box which is direct swap for the OEM oneand comes with twin rear ovals.
Changing a exhaust on even 320D does make a difference, apart from the fact the droopy one is pants and even the oval one on LCI cars is cheap and nasty and tarnishes quickly. The oval metal is very poor and looks tired quickly
Putting a less restrictive back box simply makes the car more drivable IMO. I found when on load mine sounds less strained and is just easier to drive and spool up.
F30 335i M Sport