I had a look recently at the insert for the S3 bumpers, the sensor hole diameter is 25mm, most aftermarket kits use 22mm dia sensors so they wil not fit right (they can be fitted with some sort of 'shroud' I suppose but would look tacky IMO). Don't have part number with me but will dig it out when I get home.
Also, I remember that the contour of the insert is slightly different to the standard S3 insert so the that sensors sit horizontal when fitted, it has a kind of bulge across the centre line and the sensor holes have mounting points in the rear of the insert too.