Will these diffusers interfere with the stock trunk seal or rear bumper clips?

These are designed to mount to the lower bumper panel without contact with the trunk seal or latch mechanism. Verify clearance against your bumper's existing mounting points before final installation — diffuser depth is minimal and shouldn't require bumper removal.

Should I drill through the bumper or use adhesive to mount these?

The mounting method depends on your bumper's construction and your preference for permanent versus reversible installation. If your bumper has pre-drilled holes or mounting bosses for aftermarket aero, bolting is the most secure option. For clean aesthetics without drilling, high-temperature automotive adhesive rated for carbon fiber works well — just ensure the bumper surface is thoroughly cleaned and sanded before application.

Do these stay stable at highway speeds?

Carbon fiber's rigidity and light weight resist flex and vibration under sustained airflow. Proper mounting — whether bolted or adhesive — is critical to stability. Ensure all fastener points or adhesive seams are fully secured before highway use, and periodically inspect for any movement.