Note that according to a friend of mine, who is the General Manager for one of the Disney operated restaurants, the restaurants get a fixed amount for the meal, no matter what is ordered, and the server's tip is based on that amount. The server's are aware of this. They are instructed to not suggest to the guests that they order the most expensive things on the menu. Under a previous plan the restaurants were reimbursed on the bill, and so servers benefited by having the Guests order expensive items.
If you feel that you have received excellent service you should tip more, just as in any dining experience.