Sift together flour, salt, and 3 tablespoons sugar; set aside. In a large bowl, whisk together yolks and zest; whisk in lime juice and buttermilk. Add flour mixture and whisk until blended.
Bring a large pot of water to a simmer. Meanwhile, in the bowl of an electric mixer, beat egg whites until frothy. With mixer running, gradually add remaining sugar. Continue beating until soft peaks form (when you lift whisk, tips of peaks should fall back down), about 7 minutes. Gently fold egg whites into yolk mixture, one-third at a time, until just combined.
Scooping from bottom of the bowl (so each serving will have sufficient Key lime juice), divide mixture evenly among ramekins. Carefully fill pan with enough simmering water to come halfway up sides of the ramekins. Coat a piece of foil with cooking spray; place over ramekins, greased side down. Bake 25 to 30 minutes, or until soufflés are firm and have risen just above ramekin rims.
Transfer pan to a cooling rack and let sit, uncovered, 20 minutes (soufflés will collapse slightly). When cool enough to touch, remove ramekins from pan to rack to cool completely. Serve garnished with berries, if desired.