Don't know about the zetec since it's "uncharted territory" but for example on my thunderbird, it came with dual 2.25 from the factory. My 3" single exhaust really added a lot of power, probably 8-10hp, but the major benefit wasn't the power boost, it was how much it cut the turbo lag.
I would, with the stock exhaust, see full boost around 2900-3000rpm; with the 3" it was at full boost by 2200-2300rpm. That made for a huge performance and drivability gain.
So, yes 2.25" will probably be sufficient, but in the future 3" would be a definate upgrade.
