Preheat oven to 425°F.
Butter four 1 1/4 cup souffle dishes.
Melt 2 tablespoons butter in large saucepan over medium heat. Add onion and leek; saute 3 minutes. Mix in 1 1/4 cups milk and 1/2 teaspoons salt, then grits; bring to simmer. Reduce heat to low, cover and cook until thick, stirring occasionally, about 5 minutes.
Meanwhile, whisk remaining 1/4 cup milk and yolks in small bowl to blend.
Remove grits from heat. Stir in yolk mixture, then green onions and 3/4 cup cheese.
Beat white in medium bowl until stiff but not dry. Fold into grits in 3 additions. Divide mixture among prepared dishes (mixture will come all the way to the top of dishes). Sprinkle with 1/4 cup cheese.
Bake souffles until puffed and brown on top, about 18 minutes. Serve immediately.