Got the Legendary Sparrow you can get by per-ordering the expansions, and I can't use the drives on it. Apparently rare Sparrows are better then legendarys... So that's stupid. *claps*
Edit: So I think there should be a way to upgrade legendary sparrows to an exotic status. You can upgrade rares, so why can't you upgrade legendarys?
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So to recap... The raid and dlc legendary sparrows have AFAIK the same base speed and durability as a rare (bought) sparrow. What makes them legendary is their special perks (tumbling or the weird boost on the raid sparrow) and a colored combustion-flame. You can't upgrade a legendary raid/dlc sparrow to legendary (self-explanetory but here goes) with a Xur upgrade. A rare (bought) sparrow can be upgraded and will recieve 1). A speedboost 2). Enhanced durability and 3). Colored combustion-flame. That's how it works boys and girls, It isn't speed nor durabilty that make a sparrow legendary but special abilities and a colored combustion-flame!