Originally it was my intention to do just as above and have a removable cat with flanges befor and after.
This is whats on mine its a 200 cell weld in jobby with the best available type of honeycombe, forget the name. Done over 300 miles no eml.
Replaced flexi while i was at it. One thing Ashley said was bolt up the downpipe and cat to exhaust and tack weld the flexi. If not pretty much guaranteed it not lasting two weeks. Also stated they have no faith in flexi joints.
When i get the welder running ok i will make a complete downpipe up in 2 1/4 or 2 1/2". So could use a three bolt flange and two male and one female two bolt flange.