For peach sauce: Combine peaches, water, brown sugar, and Georgia Peach Spice in a medium pot. Cook over medium heat, stirring frequently, for about 20 to 25 min., or until very soft. Mash peaches with a potato masher. Let cool completely.
For topping: In a medium bowl, combine flour, brown sugar, and Georgia Peach Spice. Add butter and use your fingers to rub the butter and dry mixture together until combined and crumbly. Stir in nuts and water. Refrigerate crumble until ready to use.
For cake: Preheat oven to 350 degrees. Grease a 9x13 baking dish and line the bottom with parchment paper. In a medium bowl, whisk flour, baking powder, baking soda, and salt. Using an electric mixer on medium-high speed, blend softened butter, sugar, and Georgia Peach Spice until light and fluffy, about 2 to 3 min. Add eggs, one at a time, and beat until combined. Mix in 1 1/2 cups of cooled peach puree, sour cream, and vanilla extract. Use a spatula to fold in flour mixture, just until combined. Pour batter into prepared pan and spread evenly. Spoon remaining peach puree in dollops onto cake and gently swirl the puree into the top of the batter. Bake for 15 min., then scatter the reserved streusel topping evenly over the top of the cake. Bake an additional 30 to 35 min., or until topping is golden brown and a toothpick inserted into the center has only a few moist crumbs sticking to it. Let cake cool for at least 1 hour before cutting. Serve warm or at room temperature.