In a small sauce pan combine the sugar and water and bring to a boil. Swirl the pan until the sugar has dissolved completely. Pour the simple syrup into a small heatproof container, and let cool in the refrigerator. (If you're in a hurry, put it in the freezer to cool.)
In the bowl of a food processor, pulse the mascarpone with 1/3 cup of the simple syrup. Taste for sweetness and add a little more syrup if you like. Add the lemon juice to the mascarpone and pulse to combine; don't overwork it or it will curdle. Adjust the simple syrup and lemon juice to taste. Refrigerate the soured cream
In a bowl, toss the peach slices with 3 tablespoons of the simple syrup. Tear the mint leaves with your hands, add to the peaches and toss. Taste the peaches and add a little more syrup if you like. Let the peaches macerate in the syrup for 10 minutes.
Puree a quarter of the peaches in the food processor before assembling the shortcakes.
You'll have leftover simple syrup. It keeps in the refrigerator for several weeks and is fun to have around to sweeten fruit and use for cocktails.