Will this diffuser hang too low or scrape on steep driveways and parking lot inclines?

The Lightspeed design sits at the rear bumper profile line, so clearance depends on your current suspension height and driveway angle. Before ordering, measure the distance from your existing rear bumper to the ground at various approach angles. If you run lowered suspension, verify that the bumper itself clears before the diffuser becomes a factor.

Is this diffuser functional downforce or purely cosmetic?

The Lightspeed style generates real aerodynamic benefit through its sculpted fin design — it channels airflow and creates subtle downforce at highway speeds. That said, it's not a race-spec component; the aesthetic impact is immediate while the aerodynamic gain increases with speed. You get both.

Does the carbon fiber come finished and ready to install, or do I need to sand and clear coat it?

The diffuser arrives as finished carbon fiber. If you want maximum UV protection and gloss depth, light sanding followed by a clear coat application will preserve the weave and add durability. If you install it as-is, inspect the finish regularly for any micro-cracking under intense sun exposure — a clear coat layer is recommended for longevity.