My sensei is 8th dan. His two primary students and also instructors are both 5th dan and have been training with him for over 30 years.
They're all awesome. My sensei is more awesomer.
Faster, smoother, more powerful. Cleaner in execution, hands so fast we often have trouble seeing them. At 57 years old, there is little doubt that he can clean the clock of anyone in our school, and that's saying something, the two 5th dans are really, really, good.
I think the real answer to the question depends upon where you stand in your own training.
As a white belt, I doubt I could have told the difference between a 5th day and an 8th dan. Two years later as a brown belt, I see differences. I presume that at some point later in my training, I will perceive more subtle but important differences that will open my eyes still further.