If I have a turbo Back exhaust, do I need a decat downpipe for Stage 2?
....No, with a Turbo Back Exhaust (often abbreviated to 'TBE') you don't need a decat for Stage 2.
A TBE is the whole system without keeping the OEM downpipe. In other words, A Cat-Back piping runs back from the Cat, and a Turbo-Back piping runs back from the Turbo.
Personally, I think that unless you've got a workshop etc and are used to doing car mechanics, the hassle for Cat swopping for MOT simply isn't worth it. Especially as exhausts aren't always the easiest to remove and fit after zillions of hot and cold cycles. Also, if you fit it yourself how does this effect your warranty?