Q: Do the fins really help, or are they mostly cosmetic?

They do both. The fins were developed within the same CFD process that shaped the full aero package, where the complete system reduced drag by about 40 newtons, roughly 2.4 percent, and improved net downforce by about 534 newtons in a 30 million cell transient model. On their own the fins contribute a tidier wake and a touch more confidence at speed by keeping side flow attached longer to help the diffuser and spoiler.