Especially with a group your size, I don't really see how the deluxe dining plan can possibly be a good value. Not only are you killing yourself with tips, but the real killer is TIME. You've probably got to figure at least 2 hours for a table service meal for twelve - drink orders alone are going to take 15 minutes. ADR seating is also going to be challenging, because to seat a group that large, it probably requires that several smaller parties leave. So you probably have to expect a somewhat longer wait after ADR check-in. Three table service meals, at two hours per meal plus a 30-minute wait to be seated means spending a minimum of 6 1/2 hours/day in restaurants. Do you really want to give up that much touring time?
Meanwhile, if you want an extra table service meal or two, you can probably pay for those two meals out of pocket for less money than the DxDP upgrade for every member of the party for every day.