Arrange a rack in center of 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 remaining ¾ cup cheddar.
Bake stuffed peppers until cheese is melty, 10 to 12 minute. Let cool 10 minutes.
Top with chives before serving.
