In a large saucepan, combine the shrimp shells with the onion, carrot and 8 cups of water and bring to a boil. Simmer over moderate heat until the stock is bright orange and reduced to 5 cups, about 30 minutes. Strain and reserve the stock.
In a food processor, combine the shallots, jalapeños, ginger, macadamia nuts, coriander and turmeric with 2 tablespoons of the fish sauce and 2 tablespoons of the canola oil; puree until smooth.
In a soup pot, heat the remaining 2 tablespoons of oil. Add the seasoning paste and cook over moderate heat, stirring, until fragrant, about 2 minutes. Add the lemongrass and cook, stirring occasionally, until the mixture darkens slightly and the oil separates, about 10 minutes.
Add the reserved shrimp stock, coconut milk, brown sugar and the remaining 2 tablespoons of fish sauce. Simmer over moderate heat until the soup is reduced to 6 cups, about 15 minutes.
Meanwhile, in a large pot of boiling water, cook the rice noodles just until pliable, about 1 minute. Drain, shaking out the excess water. Transfer to 8 soup bowls.
Add the shrimp to the soup and cook until pink and curled, about 5 minutes. Season the soup with salt and ladle it over the noodles. Garnish with lime wedges and serve.