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There's no extra cup, but Reverse Mod becomes available when you complete all cups on 150cc with Gold. It's nothing special but it's good for a laugh now and then. What's really funny is not telling you're mate who's good at Mario Kart, and watch him fly off the track when the track bends the opposite way to usual... :-D
And Toads Turnpike is unrivalled by any track ever when played in reverse :-D