GRAHAM CRACKER CRUST: Combine graham-cracker crumbs, butter, and 3 tablespoons sugar in a medium bowl; mix well. Press into a 9-inch pie plate and bake until lightly browned, about 10-12 minutes. Remove from the oven, and transfer to a wire rack until completely cooled.
Lower oven temperature to 325 degrees.
KEY LIME FILLING: In a medium bowl, gently whisk together condensed milk, egg yolks, Key lime juice, and zest. Pour into the prepared, cooled crust. Return pie to oven, and bake until the center is set but still quivers when the pan is nudged, about 15 to 17 minutes. Let cool completely on a wire rack. Refrigerate until ready to serve.
Just before serving, combine heavy cream and remaining 2 tablespoons sugar in the bowl of an electric mixer fitted with a whisk attachment. Whisk on medium speed until soft peaks form, 2 to 3 minutes. Spoon over cooled pie; garnish with lime zest and lime slices if desired. Serve immediately.