My dealer actually made a point of telling me that both they and Subaru of America recommend not adding additional undercoating to the new car. Like has been mentioned before there already is protection, and any additional coating could cause more harm then good.
That being said, I really could use some better sound deeding in my Impreza. Driving on sandy, or gravel roads is load as all get out. The quite ride is one of the few things I preferred on my 2002 Jetta to the 10 year newer Impreza.