I'm a female to male transsexual and figured I would weigh in on this. I want to preface this by saying that I think MTFs who have transitioned and whose hormone levels are in the normal female range should be allowed to participate in collegiate athletics and recreational sports. If someone gets upset because a transwoman won a local road race and beat them out of a $25 gift certificate to their local running shop, then that person has some serious problems. However, elite sports in which people's livelihoods depend on getting sponsorships etc. is a different matter.
Discussions about trans people competing in athletics always frustrates me because people on both sides of the debate often have no idea what they are talking about. Closed minded bigots love to shout: "THERE ARE TWO SEXES MALE AND FEMALE AND THAT IS IT!" And "Trans women are delusional mentally ill men" blah blah blah. However, others think that anyone who questions whether or not transwomen have an advantage in sports are transphobic bigots, which is clearly not the case.
Does transitioning to female by taking hormone blockers, estrogen and getting gender reassignment surgery make you weaker, slower, and worse at sports? Of course it does! Any moron knows that. But does it completely erase the fact that the overwhelming majority of transwomen did in fact go through male puberty? The answer is simply, no, not entirely.
Although nobody cares about FTMs competing in sports, I am going to use myself as an example of what can and cannot be changed through medically transitioning. I am going to start off by stating the obvious fact that taking testosterone (for over 16 years at this point) made me a much better distance runner, and has made me much stronger physically. For a point of reference, in high school my PRs were 2:22 for 800, 4:52 for 1500, 5:14 for the mile, 11:15 for 2 miles, and 18:16 for a certified road 5k. My current PRs after transitioning (all on the road) are 16:10 for 5k, 33:43 for 10k, 1:16:50 for the half and 2:40:27 for the marathon. It's not a perfect comparison, but obviously testosterone has made me a much faster runner.
However, my dad was over 6'1, my brother is 6ft tall, and yet I am only 5'8 1/2. So, why is it that am I so much shorter than my dad and brother? And do I even need to ask this question? It's because I went through female puberty instead of male puberty beginning in late childhood/early adolescence when I was still growing and developing. Had I been able to start taking testosterone at this age, I would likely be 6'1. In fact, according to CDC height charts, a 5'8 1/2 female is in the 95 percentile for height while a 5'8 1/2 man is at the 40 percentile for height. That is a very substantial difference. Being 5 and 1/2 inches taller (or shorter) doesn't impact every single sport, but it does have an impact on many sports. I feel like I'm stating the obvious.
Are there some women who are over 6 feet tall? Of course, but had that same woman gone through male puberty instead of female puberty, she would likely be over 6'5.
In addition to a roughly 5-6 inch difference in height, there is also a difference in the shape and structure of pelvic bones and the knee joint. I can actually feel where my hip bones and knee joints should be positioned. My brain is wired to operate a body that is just straight up and down. I was always tall and lean and I don't have wide hips or any trouble passing as an average straight white guy, but if someone found my skeleton, they would look at my pelvic bones and conclude that I was female. It sucks and I would do anything to change this, but unfortunately, there are anatomical differences that cannot be altered after having finished growing. I will never have the body that I should have been born with and that is the sad reality that almost all transsexuals face.
The real question to ask is not whether or not someone is XY or XX , but whether or not an individual went through male puberty during adolescence. If an MTF transitioned at a young age then I don't see any reason why would have any advantage over women who were born with all of the correct parts.
I absolutely consider Male to female transsexuals to be women, but almost all of them had to experience the horror of going through the wrong puberty, which altered their bodies in ways that transitioning cannot entirely reverse. Just my 2 cents.