Thanks for the comments. I learned a few things in summary:
The US version,
18100443581 includes the centre pipe. You can see picture of the US version showing both pieces in the
DIY thread.
As described by Marto323i the Euro version,
18100443580 only includes the rear muffler and a clamp for 63.5mm OD pipe. Officially BMW do not promote it as being suitable for 2.5 litre engines. I have no understanding of the reason.
My 325i dosen't have any secondary catalytic converters in the standard exhaust system, see the pic attached. This is unlike the US spec cars such as 328i.