Decat - more than likely a mot fail , unless you know someone that will mot the car
100cell race cat - passed mot on mine (not sure if anyone else has had problems ? )
200cell sports cat - passes mots
All of these downpipes will increase flow in your exhaust causing less backpressure on the turbo ( more power, a bit more of a grumble from exhaust) , however some of them will be MOT fails or likely MOT fails.
A deacat would be the cheapest
100 cell cat is med price
200 cell in slightly higher agian.
To get the best gains out of an exhaust you are best getting the car remapped aswell ... If you do get the car remaped you will need a revision g diverter valve as they usualy fail with increased boost.
Hope this helps.