Will this diffuser hang lower than the bumper and scrape on driveways or steep inclines?

Diffuser rake and clearance depend on your ride height and suspension setup. Measure the lowest point of your current rear bumper, then reference the diffuser geometry against your typical approach angle. If you've lowered the vehicle significantly, test the fitment at your driveway grade before final installation.

Does this add functional downforce or is it purely cosmetic?

This is a styling diffuser. While the shape echoes aerodynamic form, functional downforce requires tuned undertray airflow, which depends on full rear-end treatment and overall vehicle setup. Treat it as a visual enhancement that complements a Bulba-style theme.

What should I do to prepare the surface before painting?

Sand and prep the raw FRP surface with appropriate abrasive media, then apply your chosen basecoat and clear. The exact grit and primer layup depend on your paint system — follow your coating manufacturer's surface prep guidelines to ensure adhesion and finish quality.