Of course you ask: 10% improvement in
what?
The Advanced Design Director (Europe) simply
says: "...improves aerodynamic efficiency by up to 10%." So I'm assuming he's referring to Cd.
The roof extension slides back by nearly 10cm at highway speeds (and also tilts up under hard braking). There's also an active spoiler/diffuser under the bumper:
What's interesting is that there appears to be gaps between the extensions and the surface they extend from, creating slots. Presumably airflow would be drawn through the slots, top and bottom, by the lower pressure behind the car.