Will the diffuser scrape on steep driveways or ramps?

The diffuser adds minimal extension below the factory bumper line. Check ground clearance on your vehicle at full compression before installation, especially if you run lowering springs or coilovers. Reference your suspension setup against the bumper geometry to confirm safe clearance.

Is this diffuser functional or cosmetic?

The 3DS profile is aerodynamically inspired and visually aggressive, but serves primarily as a styling statement. Any performance benefit from rear diffuser geometry is nominal at street speeds and varies with exhaust flow design.

What prep does carbon fiber need before installation?

Sand the mounting surface lightly and clean thoroughly to remove dust and oils. Apply your chosen clear coat or protective finish before bolting down to preserve the weave and protect against UV exposure and minor scuffs during handling.