[aerohead]

We might presume that Ford actually 'tuned' the OEM diffuser for the peculiarities of the Focus' rear flow.
Some hatchbacks ended up with a 'raked', slanted chop to the roofline, and slope angles in the 'forbidden' 28-degree- to- 32-degree range, producing bi-stable flow and 'booming' as the flow jumps back and forth between fastback and squareback flow. Plastic roof extensions were added as a palliative to atone for the original sin.
I know of one variety of Focus hatchback with Cd 0.297. This suggests that Ford designers paid some 'thought' to the car.
Without a laboratory it's hard to 'accurize' modifications, and in your case, by mimicking Ford's original diffuser profile, it might have a better probability of success. Only testing will tell.