Many times in this forum people have found that the OEM Catalytic Converter produces the most horsepower from the ST engine with the stock turbocharger and in some cases with the higher capacity Hybrid turbo's. Basically it's best to keep the OEM cat until you install a larger Volume turbocharger otherwise known as a Big Turbo.
I would suggest you save yourself some labor and put that new cat on a shelf until you have a Big Turbo in your hands. I will also note that the stock engine features Cast Pistons and it won't last very long once you've installed a Big Turbo. I would suggest that you take advantage of Mountunes current Black Friday sale and either order the MRX Short Block or the Long Block with the CnC ported cylinder head. If someone hasn't told you this yet I will tell you that Power costs Money. If you want 400 hp from that little 1.6 litre motor plan on spending 12 to 14 thousand dollars to get there. Because for that kind of power you'll need a new, custom, transmission, new clutch, new half shafts and probably a change over on the hubs to the Focus 5 stud pattern. It's why there are at least 2 Fiesta ST's in the Detroit area running a Focus RS powertrain. With the larger 2.3 engine 400 HP is a simple tune and you have the added benefit of all wheel drive.
