Roast potatoes on rimmed baking sheet until tender, 1 to 1 1/2 hours. Cool slightly. Scoop flesh into large bowl; discard skins. Mash hot potatoes until coarse puree forms.
Heat cream and butter in heavy small saucepan over low heat until butter melts, stirring occasionally. Gradually stir hot cream mixture re into hot potatoes. Stir in syrup, bourbon, and all spices. Season with salt and pepper.
Do ahead: Can be prepared 1 day ahead. Cover and chill. Rewarm in microwave.