I'm going to wager it's increasing drag. The air wouldn't be separating from the roof surface that early, and the spoiler actually ramps up instead of tapering down.
Find someone who owns one without the spoiler, do a tuft test, and see if the air is separating before or after the place where the spoiler is. If it's afterward, the spoiler's probably hurting your FE.
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He gave me a dollar. A blood-soaked dollar.
I cannot get the spot out but it's okay; It still works in the store
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