your cooking time will be longer, but that depends on the depth of the dish you are using. I would be inclined to use a fork or skewer and poke the potatoes after an hour or so of cooking and check if they are ready that way, maybe cook at a slightly than lower temperature so you don't burn the top of the bake. The other thing you can do is par boil the potatoes before you slice them, that will cut your cooking time down by quite a bit