Combine the sugar and water in a small saucepan set over medium heat. Stir occasionally until the sugar has fully dissolved, about 3 to 4 minutes. Bring the mixture to a gentle simmer, then remove it from the heat. If using, stir in the corn syrup. Let the syrup cool to room temperature.
While the syrup cools, bring a pot of water to a boil and prepare a bowl of ice water on the side. Score a small X on the bottom of each peach, then drop them into the boiling water for 30 to 45 seconds. Use a slotted spoon to transfer the peaches straight into the ice water. Peel, halve, and remove the pits, then roughly chop the flesh into chunks.
Add the chopped peaches, cooled sugar syrup, lemon juice, and salt to your blender or food processor. Blend on high until completely smooth, scraping down the sides once or twice as needed. For an extra silky texture, pour the puree through a fine-mesh strainer to remove any remaining fibers.
Pour the strained puree into a bowl and cover it with plastic wrap, pressing the wrap directly against the surface. Refrigerate for at least 2 hours, or until completely cold.
Pour the chilled mixture into your ice cream maker. Churn according to the manufacturer's instructions, usually 20 to 25 minutes, until the sorbet reaches a soft-serve consistency. If you don't own an ice cream maker, pour the base into a shallow freezer-safe dish and freeze it, whisking vigorously every 30 minutes for the first 3 hours to break up ice crystals.
Transfer the churned sorbet into your loaf pan or an airtight container, smoothing the top with a spatula. Cover and freeze for at least 4 hours, or until firm enough to scoop. Let the sorbet sit at room temperature for 5 minutes before scooping if it's frozen very solid.