I believe it would be borderline whether or not to fit S3 rears when you have S3 fronts.
The Cupra makes do with GTI rears and has Cupra/R32/S3 fronts.
If you do trackdays then maybe it is worth it as the S3 rears are ventilated and should help cooling the brakes and pads under heavy useage.
With grippy tyres and a BBK front upgrade, then the S3 rears are definitely worth it