post a link to the header you are speaking of and ill give you my opinion on it. whatever it is it will be better than the stock exhaust manifold and i seriously doubt that it will be anywhere close to as good as a SVT header. rule of thumb on headers for the zetec, dont waste your time on any other header except for the SVT header or one that is a clone of the SVT header because nothing else on the market comes close to it.
yes exhaust diameter makes a difference. while this isnt a 100% correct statement ill lay it out anyway and get griped at later for it. but the zetec typically makes better power the more you can open up the exhaust. it would probably make super badass awesome power if you had a header that was just 4 open pipes that went strait out the front grill with no bends whatsoever. okay that last bit might be a bit of a stretch but the basic concept is true.
it has been dyno proven dozens of times that the zetec makes the best power in open header configuration, that is with NO exhaust at all. its just illegal and very LOUD like that so therefore you have to run exhaust. so however much you can open the exhaust is going to correlate to how much power you make. the deciding factor with 90% of people however is how the exhaust sounds and how much it costs and generally 2.25 sounds best and is cheapest followed by 2.5 which doesnt quite sound as nice and tends to cost more and then followed by 3inch and bigger which sounds like ass on most N/A cars and can be ungodly expensive in most cases.
hope that answers your question
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