Q: How does the full kit compare to just adding the trunk spoiler by itself?

The spoiler alone helps air leave the decklid more cleanly, but it only manages the rear. The full kit works front to back. The splitter and canards condition air entering the underbody and add front downforce, the diffuser extension and fins organize flow underneath and along the quarters, and the spoiler finishes the job at the rear. That balance front to rear is what moves the car from net lift toward net downforce, which a spoiler on its own cannot do.