The external dimensions are the same, its the actual way the S3 gill fixes to the bumper thats different , on a sline grill there are push fit clips , where as on the S3 there are screws fixing the grill to the bumper.
If you look at the pictures of the S3 bumper I sent you , you can see the holes for the screws in the bumper
on a sline bumper there are slots for the clips of the grill to fit into.