Honestly, I think we need to have more data to say anything for sure. Specifically, I would like more info on post-op athletes and what kind of capabilities they have currently compared to both other men and women, and to their performance pre-op.
But, I can go on with hypothetical thoughts for now. If the data shows that a MtF post-op athlete performs at a general female level (e.g. can't run as fast, isn't as strong, etc), and is on the same level as most female athletes, then I'm fine with them being part of a women's division for their sport of choice. However, if they still perform at levels similar to male athletes, even if it's lower than their previous ability, then I wouldn't agree with them joining a women's division. The same goes for a FtM athlete, of course.