Destroyers are not exempt from the principles of naval architecture. While they and other warships are capable of a good turn of speed, and btw the older one's were faster-speed is no longer the priority it once was, they do so at considerable expense. They will not effectively plane, and the stern squats substantially, often on the order of twelve feet. Once the stern begins to submerge all extra horsepower is of little use, in fact, it is counter productive. "Real" figures on naval vessel top speeds is mostly conjectural, but the naval architect has as good a guess as any since hydrodynamics will always have it's day.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.