Stir sugar and water in heavy small saucepan over low heat until sugar dissolves. Increase heat to high; boil syrup 1 minute. Chill until cold, about 1 hour.
Puree plums and raspberries in food processor. Strain puree through fine sieve into medium bowl. Add syrup, lemon juice to taste and salt and blend well.
Process in ice cream machine according to manufacturers instructions. Spoon sorbet into covered container and freeze. (can be prepared 2 days ahead.) Scoop sorbet into bowls.
Garnish with plums, raspberries and mint.
