We used the plan a couple of weeks ago and were happy with our service. I always told the servers that we were using the Plan right upfront to make things easier on them as far as working the order and computers. With the exception of feeling rushed at one buffet breakfast (a meal you wouldn't expect the same level of service as at a sit-down anyway - the server was friendly, but we still felt rushed) we had extremely friendly and excellent service. Servers cheerfully explained what was included on the Plan and stayed helpful and friendly through the meal.
With a child on the Plan, telling the servers actually worked to our advantage several times. It seems that at many of the TS places, the kids are entitled to an adult appetizer - this was nice when she was especially hungry.... or when I was.... or when we just wanted to try two different things. Also, some minor items (specialty coffees) were not included.
I would let the server know upfront so that there is no confusion about what is or is not included. If you get lousy service, I doubt it will be because of the 15% but some other issue and you can always talk to a manager.