It's more likely that the rear wing is providing a bit of drag behind the centre of pressure, thus making the car more stable while it is on the ground. When it's in the air, all bets are off on the function of the wing, as it has nothing like an aerofoil cross section. When the car is descending, air comes at it from under and over the wing.
You can't scale the Laws of Physics, so the wing must be doing something, however little. Since the top guys have a range of wings to choose from, and do use different ones, it is reasonable to suppose that they make a difference.
It is also reasonable to suppose that if YOU don't notice a difference, you're not one of the top guys, and need to go faster!!!
Only joking...