Will this interfere with the trunk seal or undercarriage on a lowered setup?

The lip extends from the bumper edge, so suspension height and ride geometry matter. Before installation, verify ground clearance with your vehicle at rest and during compression. If you run a lowered suspension, physically check the approach angle with a straightedge or mock-up to confirm adequate gap between the lip tip and ground.

Do I need to drill into the bumper, or does it use adhesive?

This piece is designed for bolt-on installation using factory mounting points on the C63 bumper. No drilling or permanent adhesive is required — all fastening happens through existing OEM attachment holes.

How stable is FRP at highway speeds if there's road debris underneath?

FRP is rigid when properly mounted, but highway airflow and debris exposure are real considerations. Once installed, periodically inspect fasteners for looseness and check for cracks or delamination along the seams. Touching up any paint chips and ensuring all bolts are tight will keep it solid at speed.