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.
Titanium Silver E90 LCI 320D M Sport Saloon
- Logic 7/ USB Connectity and Bluetooth/ Visibility Pack (Xenons, Adaptive Lights,Auto Wipers Lights and Corner Lights) / Sun Protection Glass/M Spoiler / Brake Ducts / BMW Performance Pack / SS Cat Back straight through with BCS Backbox