Research has shown that after two years of HRT there are very few differences in muscle strength/bone density/etc between cis and trans women. It is up to individual athletic bodies to set standards, but requiring hormones is supported by a wide margin of trans athletes. There aren't any trans athletes dominating sports so clearly there isn't a huge advantage to being trans.
Trans athletes in the high school level obviously fall into an ethical dilemma as requiring teens to get HRT to compete is wrong. Universal standards would be nice but are unlikely.
Fallon Fox breaking her opponent's skull: all of her early opponents had awful records and weren't great fighters. Her later record was mixed, so she clearly wasn't a dominant force.