Boy howdy, that "that grill lets the exhaust sound in" rumor really needs to be stamped out. It's confusing to us newbies!
MiniGuy already explained exactly why the grill is being added: It's to allow air to enter the boot area when the rear hatch is remotely released. There's been a problem with the "vacuum" trapped down there causing the hatch to be sucked back down onto its latch.
Unfortunately, right about the same time MiniGuy let this tidbit be known, some other fella let loose with the now notorious "I bet it's to the make the exhaust sound louder!" explanation. Even though
I can't think of a plausible reason why exhaust sound that's already made its way in through several layers of sheet metal, plastic, and carpet would somehow be stymied by a flimsy parcel shelf, this explanation has caught on and is rapidly approaching urban-legend status!
Alas, like most really good stories, it's not true.
Congrats on the new S! I'm only, uh, 10 months behind you!
-Dave