The performance difference between the very best transwomen athletes and male athletes.
Not sure I'm following you. First, let's get a modifier out of the way. Did you mean for "very best" to modify "transwomen athletes", or also "male athletes".
In other words, was it intended to be the difference between the very best transwomen athletes, and the very best male athletes? Or was it intended to be the very best transwomen athletes versus the set of male athletes? I'll address both possibilities.
You said "the performance disparity reasonably leads to the conclusion that transwomen are not men."
So, taking one case at a time.
First possibility. The performance disparity between the very best transwomen athletes and the very best male athletes reasonably leads to the conclusion that transwomen are not men.
Why? It means that the very best transwomen aren't the very best men, but how can we draw any other conclusion from that? Connect the dots for me. It seems like you are saying that the distribution of athletic ability among some group, in this case transwomen, is different from the distribution of athletic ability among men, then the group cannot be men.
So, Israeli athletes have won no gold medals in any Olympic sport other than sailing, which, come on, it isn't a real sport. The very best Israeli athletes are therefore not as good as the very best male athletes. Therefore, Israeli athletes are not men? I don't think that's a reasonable conclusion, and I don't think it's any different from your reasoning about transwomen athletes.
(Aside, if you use my current definition of "men", then by definition, transwomen are not men, but I don't see how that relates to anything related to competition divisions for athletes. They aren't men, but they are male, and sports ought to be divided between males and females, not between men and women.)
Also, Bruce Jenner was indeed given the informal title of World's Greatest Athlete, and formally, won a gold medal. Laurel Hubbard was not the greatest in the world, but while competing as Gavin Hubbard, did set a New Zealand youth record, and did win events. I think Gaving Hubbard, who would subsequently be called Laurel, would be among the best male athletes.
Second possibility: The performance disparity between transwomen athletes (as a whole) and the very best male athletes reasonably leads to the conclusion that transwomen are not men.
That makes no sense. No one's performance is as good as the very best male athletes, so does that mean the only men are the very best male athletes?
I don't think we can conclude anything about who is and who is not a man by comparing athletic ability, and I don't think we ought to try.
What we could say, though, is that if a person's athletic ability is greater than the very best woman athlete, then that person is not a woman. That's a rather easy proof, but it's not useful. If we define transwomen as women, then the very best transwoman athlete is a woman, and if that transwoman is the very best woman, then there is no performance disparity between her and the very best woman, because she is a woman.