Rear sub frame is the same, even the ARB diameter is the same i belive on all Focus models. The only thing you will have to change is the hand brake cable, the ST170 has larger rear disks so the cable needs to be longer, i just fitted a brand new cable to my ST170 this weekend, cost me £47 from FordPartsUk which i thought was resonable.
The dampening rates on the rear dampers may be different so fit your rear shocks in place of the ST170 ones till you get a set. The rear springs will be 15mm lower than yours also. Get the eibach springs for definate, i have them fitted to my ST170.