In a small saucepan, combine the sugar and water and stir over high heat until the sugar has dissolved. Remove from heat and cool to room temperature. In a large pitcher, combine the cooled sugar syrup with the 1.5 cups of mint leaves and wedges of 6 limes. Muddle well with a wooden spoon, stir in the rum, and strain into another pitcher. Divide the mixture among tall glasses filled with cracked ice, filling about two-thirds of the way full. Top with sparkling wine and garnish each with a mint sprig. Makes about 10.