Does anyone know if the Fiesta ST Rotors will fit on a MK3 Focus? Also, if there is a benefit?
100% will not fit. The Fiesta ST uses a 4x108 mm bolt pattern for the lug studs where as the Focus uses a 5x108 mm bolt pattern.
Never mind the Fiesta ST front rotors are the same 10.9" as the base MK3.
The rears are 0.3" larger as it's at 10.9" all around for the Fiesta ST and 10.6" rear for the Focus and Focus ST.
On the other hand getting the caliper brackets, dust shields and calipers (essentially the front brake hardware) for a Focus ST up front you can upgrade to a 12.6" or 13.1" front rotor set up.