Will this scrape on steep driveways or inclines?

The diffuser extends below the factory rear bumper line. Before installation, verify ground clearance at your vehicle's lowest points — driveway approach angles and speed bumps are the typical concerns. Mount height depends on your suspension setup and ride height; adjust accordingly if you run lowered springs or coilovers.

Is this functional or cosmetic?

This is a styling piece designed to match the RS3 aesthetic. It does not alter airflow dynamics or add downforce; its role is visual impact. Install it for the look, not for performance gains.

What finish should I apply before installation?

Carbon fiber benefits from UV-protective clear coat to preserve the weave and prevent oxidation. Sand and prep the surface per your chosen clear coat manufacturer's instructions, then apply at least two coats. Matte or glossy finish is entirely your preference.