For those of you still interested in doing this conversion (I still highly recommend it as Mark and the guys are great!), I have some updated news for you.
We recently got a wrecked Fiesta ST into our salvage yard and I had the chance to really dive into the rear suspension and see how it differs from the standard model.
1) Obviously there is the redesigned rear torsion beam. Entirely different part than the standard Fiesta. Uses same frame mounting points.
2) We know the ST has the rear disc package. The hubs have been redesigned to work with a caliper and have dust shields. The mounting points for the ST rear hubs are specific to the redesigned torsion beam. These cannot be swapped to the standard Fiesta without swapping the torsion beam as well.
3) The rear calipers and brackets for the Fiesta ST are EXACT matches to the ones from the non-SVT Ford Focus.
4) I amend #3. There is one difference in the little eyelet that the parking brake cable goes through. It was much smaller on the Focus'. It looks as though the Fiesta ST parking brake cable has been beefed up a little over previous gens.
5) The ST specific parking brake cable will swap over beautifully to the standard Fiesta. All mounting points used are the same.
6) The Fiesta ST rear discs appear to be a match for the non-SVT Focus rear discs that I am running.
7) The Fiesta ST rear brake lines will swap over just fine to the standard model.
*) The Lapp Racing kit just became entirely complete and finished for me when I installed the Fiesta ST parking brake cable (it is MUCH shorter than the Focus one I had been running with and holds a lot firmer - and doesn't require any zipties to mount, lol), and swapped out my Focus calipers for Fiesta ST calipers so that the parking brake cable would mount properly. The SVT Focus caliper brackets still work perfectly to support the ST calipers.