If I remember, my sportscat/decatted downpipe was from the turbo (snakehead) down to the centre section.
I think the original exhaust comes in 4 parts but now I've got the front, centre and backbox.
Hope you understand my technical speak
I now have 1 x 100 cell cat. No MOT problems at all.
From my own experience, I ran my sportscat for over a year before my remap with no issues at all.
Like I said, had the sportscat, then a wee while later added the centre section and it was like that until
I got the pinks/turbo/up pipe together and then had them fitted and mapped.