I am not so sure if I agree with you're statement that the fee in the specialty restaurant is a tip for the personnel in the restaurant. I can't imagine the cruise line paying for a higher quality of food and service and not get compensated in return. I believe that a big portion goes to the cruise line and a small percentage goes to the personnal.
I don't believe for one minute that a cruise is going to let the staff keep between $60 and $80 PER COUPLE. In you're estimate then, if a waiter has 8 tables for the night at 2 per table then his take is going to be between $480 and $640 per night depending on the price of the restaurant. no cruise member makes that kind of money and if they did I would be the next one on line to apply for that job. I'm sure they will get the standard 15%-20% tops for each table they serve