In a small saucepan, combine the water, 1 cup of the sugar, and lime zest. Bring to a simmer, stirring to help dissolve the sugar, about 5 minutes. Remove the pan from the heat and stir in the lime juice. Strain the sorbet base into a nonreactive bowl and set aside to cool completely. Refrigerate, covered, until thoroughly chilled.
Pour the chilled mixture into the bowl of an ice cream maker and process according to the manufacturer's instructions. Transfer the sorbet to a lidded, freezer-safe container and freeze at least 4 hours or overnight before using.
Place a 10-inch saute pan over high heat and add the butter. Once the butter is melted, adds the strawberries, blackberries and remaining 1/4 cup of sugar. Saute until the berries are soft and the sugar has melted, 2 to 3 minutes.
Meanwhile, cut each piece of pound cake in half diagonally and place each slice of pound cake in a chilled bowl with the tips pointed out towards the rim of the bowls. Place 2 scoops of sorbet in the center of each of the chilled bowls on top of the pound cake.
Remove the pan with the berries from the heat and add the tequila. Return the pan to the heat immediately, and tilt the edge of the pan towards the fire to ignite the tequila. Pour the berries over the sorbet while they are still flaming. Drizzle extra tequila over the sorbet if desired. Serve immediately.