Would that be a backbox that comes from the Prodrive Performance Pack, or one that you can buy separate?
one will will have Prodrive on and is round and the other Has WRSport on and is usually oval, or they were.[]
just changing the backbox will not give any significant increase in performance...a better sound maybe.
P.S.
you dont say what scoob you have.
and changing the centre section gives a good result. but not as much as changing the Downpipe.