There aren't any 5 stud Fiestas.

The Mk6 Fiesta ST has ET52.5 alloys and the newer one is ET47.5 so probably not much in it really. (Both are 4 x 108)

The differences are probably just the offset as you mention. If you can get a definitive answer as to what the offsets are on that website, then you can judge whether any differences will matter. (doubt 5mm would make much problem if the 15" steels have the same ET as the ST versions)