Place shrimp shells in medium sauce plan. Add 2 cups water, bay lead, and a generous pinch of salt. Bring to boil over medium-high meat. Cover, reduce to medium-low and simmer until reduced to 1 cup liquid, about 15 minutes. Strain into measuring cup, pressing on shells to extract as much liquid as possible.
Meanwhile, heat 2 tbs oil in a heavy skillet over medium-high heat. Add shrimp and sauté until opaque, about 3-4 mins. Using slotted spoon, transfer to medium bowl. Reduce heat to low. Add remaining 2 tbs oil to same skillet. Add garlic and sauté until soft, about 1 min. Sir in leeks, peperoncini, oregano, lemon zest and hot pepper flakes. Cover and cook until leeks are soft, about 3 mins. Uncover; add wine, lemon juice, and shrimp-shell liquid. Increase heat to high and boil until reduced by half, about 2 mins. Remove from heat. Finish with butter.
Cook pasta in a large pot of boiling salted water until tender but still firm to bit, stirring occasionally. Drain, reserving 1 cup cooking liquid. Add pasta and shrimp to skillet with sauce and toss to coat. Add enough reserved cooking liquid to moisten if necessary. Season to taste with salt and pepper. Transfer to large bowl. Garnish with parsley.
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