What a difference between SVT and Fiesta brakes?
The FiST rear rotor is 253mm.
The Focus SVT rear rotor is 278mm.
So of course the rotor diameter is different, the bracket is different to space out the caliper to fit the larger rotor.
The caliper has some minor carting differences but fits exactly the same.
For these reasons you only need to change the bracket and rotor.
You will continue to use the FiST caliper and pads.
BTW: there is not enough difference between the SVT and FiST fronts to justify the cost of parts.
This is why most of the aftermarket front brake kits are much larger and/or wider.
Dave