Just want to share my experiences fitting the 2009 S3 rear valance + diffuser to my 2007 A3 standard rear bumper with bump strips.
As can be seen in photos, the valance is a direct fit and just need to put in the diffuser.
Here are the part nos for the valance and diffuser:
8P4 807 434 G 1RR - grey diffuser
8P4 807 521 K GRU - primed valance (painting required)