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!