Boil potatoes until fork tender. Drain, cool slightly, and remove skin. Beat potatoes in large mixing bowl until smooth -- should make about 3.5 cups of mashed sweet potatoes. Mix in butter & sugar. Beat in 1 egg at a time. Add remaining ingredients.
Place pie shells onto large cookie sheet. Pour mixture into unbaked pie shells. Cover edges of pie shells with foil or tin shell covers.
Bake 425°F for 20 min. Lower heat to 325°F and bake for 30-45 minutes. Pies should be firm and pass clean knife check.