Arrange a rack in the center of the oven; preheat to 400°. In a large bowl, mix cream cheese, pimentos, garlic powder, salt, cayenne, and 1 cup cheddar.

Cut peppers in half lengthwise and scoop out seeds and ribs. Spoon a small amount (about 1 Tbsp.) of pimento mixture into each half. Arrange on a baking sheet, cut side up. Top peppers with the remaining 3/4 cup of cheddar.

Bake stuffed peppers until cheese is melty, 10 to 12 minutes. Let cool 10 minutes.

Top with chives before serving.