Wash the raw shrimp, then peel and devein, saving the peelings. Wash the shrimp again, drain and pat dry. Cover and refrigerate.
Cut each lemongrass stick into 3 (2-inch) pieces, starting from the rounded bottom end; discard the straw-like top. Lightly crush the 6 pieces with a heavy pestle, a mallet, or the blade of a knife.
Combine the lemongrass, kaffir lime leaves, the stock, and shrimp peelings in a pan. Bring to a boil. Lower the heat and simmer very gently for 20 minutes. Strain this stock and return to the rinsed-out pan.
Add the fish sauce, lime juice, and chile paste. Stir and taste again, adding more fish sauce or lime juice if required.
Drain the straw mushrooms and add them to the seasoned stock. If using fresh mushrooms cut into quarters, drop into lightly salted boiling water, and boil for 1 minute. Drain and add to the seasoned stock. (The soup may be prepared to this stage several hours ahead if necessary and refrigerated.)
Prepare the garnishes shortly before you are ready to serve the soup. Cut the green chilies into very fine rounds, and wash and dry the cilantro leaves.
Just before serving, heat the stock with the mushrooms in it. When it begins to bubble, drop in the peeled shrimp. Cook over a medium heat for about 2 minutes or just until the shrimp turn opaque. Serve the soup immediately in individual bowls, garnished with the green chiles and cilantro leaves.