That must have been the worst analogy I've ever seen in my life...
Sishui is known for body flicker, Do you know why? Is it speed? his skill? Does he has a unique way of doing it.. We can't assume anything like that,,
And even if it was for speed..
Understand this one concept. Nothing is faster than space/time. When you understand that, you'll understand why your logic is wrong. Shishui could have very well been faster than base minato.... But he was not faster than minato...Not only that but juxtaposing something as vague as a moniker with someones confirmed speed, Is no where near the same as using manga statements that state a specific thing..
For eg, Sakumo hatake and Kakashi.
If Tsunade stated that whitefang was faster than kakashi... What's there to dispute really? This is hop kishi uses hype in the manga.. Not everything is correct... But when Sunagakure elders say that the third kazekage was by far the strongest kazekage they ever had.... Would you deny that?
Itachi never reached his prime.. Prime is like 40.. Itachi was only 21.. him and Minato were young and still had potential to learn more jutsu and get stronger..
If you are referring to itachi's prime state between birth and death, then his prime is still not really guessible, since he was plagued with sickness, and at 21 was a walking dead man..