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The 996RSR of Konrad-Lechner and Alzen had underbody CF panels during last week end's 24hours in Dubai. They were second and third behind the Red Bull Z3GTR. I guess the aerodynamic benefits and lift avoidance do have more benefit than fresh air.
You're also ignoring the differances between a car driven on a track only, at very high speeds vs. cars that are driven much slower and frequently in (hot) stop and go traffic. If I drove nothing but Autobahn speeds I would leave both trays on all the time too. Unfortunately, I don't.