With the 3L or FI... or both, the Zetec final drive makes the engine *grunt* harder down low before the tires spin. This aids in launching the car more steadily, plus makes better use of the power at hand. The SVT 2.5 would not have enough low end to need this extra gearing in most cases.
Way to tell if you need it? If you think first gear is useless and only makes smoke before you shift to second (without dumping the clutch!), then you can probably make use of the taller gear.
