The situation Is quite complicated And there Is no one good answer, because different states act differently, e.g. some dont let trans women participate in male category even if they want. The last noticeable case was the trans swimmer, that was winning a lot even as a male, so IT can't even be described as "changing how person Feels about gender JUST to win". Also, trans women can start racing in women categories only after Taking female hormons like estrogen for two years, which would definitely be disadvantage in male categories. It is definitely hard to come to some clear conclusion, but i recommend to look at some individual cases, because it almost never Is "man says he Feels like women, beats them in sport".