Ah OK I'm perhaps not using the right terminology to describe what I mean.
By "cat back" I think I mean the rear muffler section, on the e92 M3 this was available excluding the mid section as a slip on to the existing pipe work. (the Tubi Rumore was about 4 feet long)
Like this:
The MPE and Akra are far too rich for me I couldn't spend that on an exhaust £7,000 + fitting in the UK.
It's just the mufflers I want to change and from what I've read here that will require cutting.
Sorry if I'm reading this wrong and thanks for the help!