Will this scrape on steep driveways or parking lot inclines?

The V3 diffuser adds extension to the lower rear, so clearance depends on your suspension setup and approach angle. Measure the ground clearance of your current bumper, then account for the additional panel depth before installation. If you run lowered suspension, verify clearance on your regular routes.

Is this diffuser purely cosmetic or does it have functional downforce?

The RBS V3 design incorporates diffuser vanes that work with airflow at highway speeds, but the primary benefit is visual impact and chassis styling. For measurable downforce gains, this functions best as part of a complete aero package rather than as a standalone component.

What prep work is needed before painting?

Sand the surface with appropriate abrasive to establish a uniform base, then wipe clean before applying your chosen primer and topcoat system. Follow your paint supplier's surface preparation guidelines for best adhesion and finish quality.