OK - so you want to see the end result - Well here's a few shots to keep you happy:
Basically I opted to go from cat back and replace with custom bent stainless and all warranted for lifetime

. The guys at MIJ flanged the new mid pipes, so the correct ///M sport back box clamp fittings can be used. Also eliminated the central resonator that improves flow, It also makes it a separate system for cylinders 1-3 and 4-6, which they say is just fine and itself promotes a different sound. Perhaps not clear from the photos, but I managed to get the OEM heatshield in, modified to protect the side of the new battery box too and modify all the underpinning fittings to keep airflow correct.
The sound is awesome and by that I mean no excess loud noise, no drone, just a meaningful growl when I hit the gas and get the revs going, that fades as soon as I throttle back.
Few tidying up bits to go so ignore the 'wave' in the centre of the diffuser. I have to smooth the rear of the battery box off to accomodate the diffuser profile that at the moment is twisted slightly by the bolt to location. I also need to decide if I should paint the lower edge of the boxes matt black so they blend in (They are brand new, so a bit too shiny) and finally seal the diffuser to bumper cover.
Project met my criteria of being OEM look (how BMW shoudl have done it) and well engineered, fully functional and working (the GP20 test of all 4 steaming...

), no loss of power nor gain really sought, not excess noise/drone and fitted without loss of bootspace and no net weight gain.
Will get a better shot with the diffuser lit, but here's a few for now: