The roll bar is solid and well made... Big drawback is that the areas where the bar mounts to the floor, are not a pinch weld area or structural point, and very flimsy. For the bottom plate I would strongly recommend a larger plate not nearly as thick (thinner is actually a much better idea for this)... that's stitch welded to the pan, and extending to the structurally supported areas and seams for at least 2 edges if not 3, and then have the standard backing plates still used outside of that as designed.
My thoughts are that the Fiesta car structure is perhaps good at a 50mph rollover scenario (autocross) and the roll bar gets you an additional 50mph (which is not much at all, perhaps 70-80% of the race track for a time trial or HPDE) at most with a helmet and respecting head strike issues, a properly integrated cage is needed for above that and any door to door of course... just rough spitballing here, but the biggest issue is the additional energy would poke through the unsupported floor first and crush the A pillar later, depending on the type of crash obviously. My concern was the high CG and sticky tires there's a legitimate rollover risk. I remember even just parking at an autocross event parking lot style, and the little Fiesta was the tallest car in the lot.