Yes, Shishui could have been very strong, but almost everything you are saying is speculation. He had body flicker and a unique sharingan, we know this.
But "I think he would have got MS" - 100% opinion
"He was the strongest Uchia" - 100% opinion
What do we know? We know that it has been stated that Itatchi was the pride of the Uchia and one of the strongest they had seen and was a ANBU member from such a young age. We know that Madara was at one point the second strongest in the world and had the power to control the 9 tails - so its just stupid for you to say that you think Shishui could have surpassed both just because you "think" so.
There is a lot of evidence showing that Itachi was stronger than him, but almost none showing the reciprocal.
Conclusion? No proof, 100% speculation.