Graham cracker crust:
Preheat oven to 375F.
Make the graham cracker crust. In a small bowl, combine 1 1/2 cups graham cracker crumbs, 1/3 cup granulated sugar, and 6 tablespoons melted butter. Stir until the crumbs are evenly moistened.
Press the crumb mixture firmly into the bottom and up the sides of an 8- to 9 1/2-inch pie pan.
Bake the crust for 7 minutes. Remove from the oven and let cool for at least 30 minutes.
Reduce the oven temperature to 350°F.
Make the key lime filling. In a medium bowl, whisk together 28 ounces sweetened condensed milk, 1/2 cup light sour cream, 3/4 cup key lime juice, and 1 tablespoon lime zest until smooth and well combined.
Pour the filling into the cooled graham cracker crust and spread it evenly.
Bake for 10 minutes, or until the filling is just set. Do not overbake.
Remove the pie from the oven and let it cool slightly. Cover and refrigerate for at least 3 hours, or until completely chilled and set.
Make the whipped cream. Chill a mixing bowl and 1 cup heavy whipping cream in the freezer for about 15 minutes.
Add 2 tablespoons granulated sugar to the chilled bowl. Pour in the cold heavy whipping cream and beat with an electric mixer until stiff peaks form.
Spread or pipe the whipped cream over the chilled pie. Garnish with additional lime zest, if desired.
Slice and serve chilled. Refrigerate any leftovers.