Based on the testing I've seen (I believe the video has been posted on the forum) none of the black tank flush systems are going to get everything out of the tank. My prior TT didn't have one and after six years of filling, flushing and putting two gallons of water in it with a packet of deodorizer after dumping it remained functional and the trailer odor free for six years. There may be some benefit in using a flush system that sprays the sides of the tank and helps keep the sensors clean but as for attempting to get every last bit of solid material out of the tank is concerned I'm doubtful they can accomplish that task.
At our regular campground the checkout is 11:00am. My wife and I pay for an extra day since we'd rather check out late afternoon, not have to get up early and rush out of the campground with everyone else. The side benefit is there is usually no one at the dump station when we get there at 3:00pm ~ 4:00pm.