I've always used a dining plan but service has always been good, and often fantastic... at all restaurants - character meals, signature restaurants, breakfast, lunch, dinner, etc.
Once at a signature restaurant... the waiter offered for DS to order from the regular menu instead of the kids menu (including regular appetizers) and other extras... but included it under the dining plan. It did make the amount the tip was based on larger... but still all covered under the dining plan credit. I would not expect that, it's not the norm.... but most signature restaurants were more than happy to make adjustments for DS if he wanted something different.
Example --DS often substituted a bowl of Florida strawberries for an appetizer and the same for dessert in place of the usual items on the kids' menus.
My experience is waiters and waitresses want guests to be happy. Happy guests = bigger tip.