Will this diffuser scrape on steep driveways or parking lot inclines?

The Rega diffuser extends modestly from the rear bumper. Before final installation, verify ground clearance at the lowest point by measuring from your tire to the bumper, then add the diffuser's extension. Most Z4 owners find standard approach angles manageable on typical roads, but steep or sloped entries may require a test fit first.

Is this a functional diffuser or purely cosmetic?

This is a styling piece designed to enhance the rear end's visual aggression and proportion. While FRP diffusers can contribute minor aerodynamic benefits at highway speeds, the primary purpose is aesthetic. It does not alter downforce or significantly impact performance.

What prep is required before painting?

Sand the surface to promote adhesion, fill any imperfections if needed, and wipe clean before applying your base coat and clear. Work with your preferred shop or painter to ensure color and gloss match your bumper. The quality of your prep directly affects how seamlessly the diffuser integrates with the rest of the rear end.