The cossie rear are still only 273mm(vented on 4wd).
The abs sensor on the cossie picks up on the driveshaft, it has a toothed section for this, your abs may not need touching depending on where the pick up registers, if on a bit that stays on the car with escos discs calipers fitted, which is what I suspect/expect then all will be well.
Would you notice any difference from 3mm ?, I suspect not, you also have the height difference of the two discs which is around 25mm, so would need spacing etc.
You would get vastly more effect by using a decent set of pads, Reds or yellows, but MUST be done both ends of the car at same time, or will be unbalanced braking.
Even the Focus RS(mk1) is only 280mm rear.
tabetha