Will this diffuser scrape on steep driveways or inclines?

The diffuser extends below the factory M Sport bumper line. Check your driveway approach angle and any speed bumps on your regular routes before installation. If you frequently encounter steep inclines or low-angle entries, measure the bumper-to-ground clearance at your vehicle's current ride height.

Is this functional downforce or purely cosmetic?

This is cosmetic. It captures the M Performance aesthetic of the factory treatment. It does not generate aerodynamic downforce — it completes the visual package of the M Sport rear bumper.

What do I need to do before painting?

Sand and prep the surface with appropriate grit per your paint system's specifications, then apply your chosen basecoat and clear. Consult your paint supplier's prep guidelines for the specific primer-to-paint sequence they recommend.