Special equipment: a 1 1/2-quart ice cream maker
Combine the water and 2 cups of the sugar in a small saucepot. Bring to a boil over high heat and cook until the sugar is just dissolved. Remove from the heat and let cool completely.
Set a strainer over a 2-cup liquid measure.
Cut off the top quarter of 8 lemons, then cut a very thin slice from the bottom to make a flat base. Squeeze the juice from the lemon tops into the liquid measure; discard the tops and bottoms.
Stand up a cut lemon and, using a paring knife, cut around the inner edge of the peel, so you cut the flesh away from the peel. Switch to a spoon and use the incision as a guide to scoop out all the flesh from the inside of the lemon, leaving behind a lemon "cup." Place the flesh in the strainer over the liquid measure and squeeze out all the juice. Repeat with the other 7 lemons. Freeze the lemon cups until ready to serve.
Zest and juice the remaining 4 lemons; reserve 1 tablespoon zest for the topping. You'll need 2 cups of fresh lemon juice in all; if you have any excess, store it in the fridge or freezer for future use.
Mix together the cooled simple syrup and the 2 cups lemon juice. Pour the mixture into an ice cream machine and freeze following the manufacturer's instructions. Remove the sorbet to an airtight container and hold in the freezer for at least 2 hours before serving.
On a small plate, mix together the reserved lemon zest and the remaining 2 tablespoons sugar. Spread the mixture in an even layer and set aside for at least 30 minutes to dry out slightly.
When ready to serve, scoop the sorbet into the frozen lemon cups and top with sugared lemon zest. Serve immediately.