Debateable to be honest...
Should be performing partially as a "ducktail" type spoiler, as it pokes up just high enough to catch the flow coming over the roof, although the radius/hump in the roof line will be probably slowing the air sufficiently for it to be then negligable. But if it is itll bouce that air back off the top of the car and stop it swirling with the draft at the back and the compressed air rushing from beneath the car and thus give marginal stability to the rear at high speed... similar to the recall with the Audi TT when they had to fit the ghey little spoiler to the back to stop the ass end getting light on the motorway.
However the attack angle is nothing to shout about so it wont be giving any downforce in anyway shape or form.
I have kind of noticed the airflow lines in the crap thats building up on my car lol and it seems to help keep the rear screen clear of dirt... ive had no end of hatches that just end up with a swirling pocket of doom at the rear of the car, plastering the rear window with road guff.
Would be interesting to see one in a wind tunnel.