Same thing would happen as if they entered the ring with a 5'8" 13 stone man. Absolute kicking. So what? Going up against a genetic outlier has its risks, unless you're also a genetic outlier.
On the other hand, how many times has it actually happened? Is it really likely to happen? Are you genuinely concerned for those the poor female heavyweight boxers, or are you just using the argument as a stick to beat all transsexuals, because you don't agree that they exist?
Ask yourself whether it's more likely that Manu Tuilagi decides "you know what, I'd rather be the undeniably world's best female rugby player, than one of the world's best male players, I'm 'just going to announce I'm transitioning' in order to game the system" - or that she decides that after decades of depression and struggling to cope with being trapped in a man's body, that she decides to transition; but wants to carry on playing the game she loves at whatever level, after meeting all of the criteria for hormone levels, and undergoing reassignment surgery?
There are a lot of females who dont want transgenders participating in the sports.
Are they all wrong?