To make the Mushrooms: Soak porcini in boiling-hot water, stirring occasionally, until softened, about 15 minutes.
Meanwhile, wash leeks.
Strain porcini through a paper towel lined sieve into a bowl, reserving liquid. Squeeze liquid from mushrooms into sieve, then rinse mushrooms well to remove any grit and chop.
Cook leeks in butter in a 12-inch heavy skillet over medium heat, stirring occasionally, until tender, about 8-10 minutes. Increase heat to high, then add all the mushrooms, reserved soaking liquid, and wine and boil, stirring occasionally, until mushrooms are softened, about 5 minutes.
Strain through a clean sieve into a liquid measuring cup, pressing hard on and reserving mushrooms.
Return liquid to skillet and boil it until it is reduced to 1/3 cup, about 5 minutes. Add cream and boil over medium-high heat, stirring occasionally until thickened, about 3 minutes. Add mushrooms, 1/2 teaspoon salt and 1/4 teaspoon pepper, and boil, stirring, 1 minute. Transfer to a 3-quart shallow baking dish.
To make the macaroni: Preheat oven to 400 degrees with rack in the middle.
Cook macaroni in pasta pot of boiling salted water until al dente. Drain and rinse with cold water to stop cooking, then drain again.
Meanwhile, heat milk in a small heavy saucepan over medium-low heat until hot.
Melt butter in a heavy medium pot over medium-low heat, then whisk in flour and cook, whisking, 3 minutes.
Whisk in hot milk, Worcestershire sauce, mustard and 1/4 teaspoon pepper and boil over medium heat, whisking constantly, 2 minutes, then whisk in 3 cups cheese until melted.
Remove from heat and stir in macaroni and 1 1/2 cups cheese. Spread over mushrooms.
Sprinkle evenly with remaining 1/2 cup cheese. Bake until golden, about 30 minutes.