Pierce each potato twice with point of sharp knife to vent steam. Place in single layer in microwave oven. Microwave on High 5 minutes. If potatoes are not cooked through, turn potatoes over and continue microwaving 1 minute at a time until softened. Remove any fully cooked potatoes from microwave while cooking remaining potatoes. Cool potatoes 10 minutes.
Meanwhile, in small bowl, mix flour, brown sugar, 2 tablespoons butter and the pecans; set aside.
Heat oven to 350°F. Cut off top 1/3 of each potato. Using spoon, scoop flesh into large bowl, being careful not to tear potato skins; discard potato tops. Add 2 tablespoons butter, half-and-half, pumpkin pie spice and salt to potatoes. Mash potato mixture with potato masher or beat with electric mixer on medium speed until creamy. Spoon potato mixture back into skins. Place filled potatoes in 13x9-inch pan. (Potatoes can be covered and refrigerated at this point up to 8 hours.)
Sprinkle pecan mixture over tops of potatoes. Bake uncovered 30 to 35 minutes or until topping is brown and potatoes are hot. (If filled potatoes were refrigerated, remove from refrigerator 30 minutes before serving; top and bake as directed.)