GCDs are 1.5 seconds. To make a full stack, that requires 7.5 seconds. We'll say you use 4 instead of five and make your first attack on the 5th, so you spend 6 seconds to kill 10% faster. That would mean that you need your fights to last near a minute for this to be more efficient than fighting with a normal rotation.
Maybe it's not mathematically correct, but it's the idea of it, and I doubt its worth as well.