Place the noodles in a medium bowl and cover with enough cold water to completely submerge and soak for 30 minutes. Then, drain and set the now pliable noodles aside.
Remove the heads and shells from the shrimp and place in a medium pot (the heads and shells that is). Split the shrimp completely in half, lengthwise, remove the vein, and set aside in a small bowl.
Mash the shrimp heads and shells with a potato masher or heavy wooden spoon, then add the 5 cups of cold water and put over high heat. When the water hits a boil, reduce the heat to low, and partially cover the pot with a lid. Simmer for 30 minutes then strain through a sieve into a medium bowl. Set the stock aside and discard the heads and shells. Reserve 1 cup of the stock for the Pancit and save the remainder for another dish. (Shrimp stock can be frozen for up to 6 months and is wonderful when cooking grits...as long as there are no allergies, of course.)
Combine the fish sauce, hoisin, oyster, both soy sauces, and the lime juice in a small bowl and have standing by the cooktop.
Saute the lap cheong in a large wok over medium heat to render the fat and crisp the sausage, about 7 minutes. Remove with a slotted spoon to a small bowl, leaving the rendered fat in the wok.
Boost the heat to medium-high, add the shrimp to the wok, and saute for 2 minutes. Remove to the bowl with the sausage.
Add the vegetable oil to the remaining fat left in the wok along with the shiitakes, garlic, and onion and cook for 3 minutes. Follow with the cabbage, carrots, and snow peas and cook another 3 minutes. Add the cooked sausage and shrimp along with the fish sauce mixture and toss to combine.
Add the drained noodles along with 1 cup of the shrimp stock to the wok and gently toss to combine with vegetables. If it seems to be a little dry, add more stock, a tablespoon at a time, until noodles are cooked through and nicely coated.
Serve immediately, topping with the sliced scallions and a few squeezes of lime juice.