The original Catalyst and Downpipe of the New Mk7 Fiesta ST180 EcoBoost are made in one piece. This new one piece design features a restrictive bend and very small 48mm internal Bore (under 2 inches!). This 48mm small Bore is no where near the optimum flow rate of the Fiesta ST180 Borg Warner Turbocharger. 
you have the chioce of the sports catalyst or the straight through De-catted pipe