Well, something you need to consider in sizing an exhaust is where you're wanting to focus the powerband. The size of you're piping will have an effect on velocity and scavenging, smaller piping will create optimum velocity and scavenging at lower rpms, while becomeing restrictive up top, a larger exhaust won't produce good effect on the lowend, but will breath better up top. Too large will simple hurt power all around. Its the same concept as with the Duratecs intake design, we have two different runner to create the optimum velocity at a variety of RPMs. Usually when sizing and exhaust, its about finding a happy medium.
The difference between a 2.25 and a 2.5 is probably negligable on a mildly modified Duratec. The motor will absolutely love 2.5 inch better though, as its power, while broad, is focused in the upper rev ranges, especially the SVT.
