Cook polenta according to package instructions in a heavy medium saucepan until thickened and creamy, about 5 minutes.
Remove from heat and season with salt, then cover.
Cook pancetta, garlic, and red pepper flakes in 2 tablespoon oil in a 12-inch heavy skillet over medium heat, stirring, until garlic is pale golden, 2 to 3 minutes.
Add tomatoes with their juice and simmer until liquid is reduced to about 1/4 cup, 6 to 8 minutes.
Add shrimp and cook, stirring occasionally, until shrimp are just cooked through, about 3 minutes. Season with salt.
Spoon polenta into shallow bowls and top with shrimp mixture.
Drizzle with remaining tablespoon oil, season with pepper, and sprinkle with parsley.