Will this diffuser scrape on speed bumps or steep driveways?

Clearance depends on your rear bumper height and suspension setup. Measure the gap between your current bumper and ground, then verify mounting bracket geometry against your vehicle's approach angle. Most rear diffusers sit flush with the bumper, but driveway angle is the real test — take it slow through inclines until you're confident in your clearance.

Is this functional downforce or purely cosmetic?

This is functional geometry — the three-piece design includes actual diffuser channels that manage airflow under the rear bumper and generate downforce at highway speeds. It's not a cosmetic splitter bolted to the bumper. Real benefit kicks in above 50 mph; below that, it's mostly visual styling.

What finish should I apply, and do I need to sand it first?

Sand and prep the surface lightly before applying your chosen clear coat or paint — this gives the finish something to grip. Carbon fiber can take a UV-protective clear coat alone for the raw weave look, or you can paint it to match your bumper. Either way, surface prep is critical for longevity against UV and environmental exposure.