Originally Posted by S0C0nFused
The most restrictive part of any exhaust is the cat. Remove it/them and the rest is comparatively fine tuning. You may also look at it the other way. Mounting a header with a trailing cat, gain would be effectively zero, as any potential gains from the header would be negated by the cat.
Not necessarily true. This all depends 100% on the cat. I'm from the 1.8T/VW world originally, and we've done tests with a 700+hp 20v motor that with the OEM VW cat vs a straight downpipe. The cat caused an effective loss 7hp on an engine dyno, which would be even less at the wheels. If you're pushing even less air through the motor, you're not even going to see THAT much loss. However, as I am fairly new to the Focus world, I don't know what the situation is with the factory cat. If you wanted to pony up for an appropriate high quality metal substrate cat, you would probably not lose anything. Especially since a mildly modded 2 or 2.3L n/a motor isn't flowing all THAT much in comparison to a forced induction motor. The main reason to not run a cat is either because of cost of putting a proper one in, or the complexity of the install making it unfeasible.