I don't think you have much hope of having a distinction between treatment and enhancement unless you have the notion of the normal state or proper function of the human body. I previously thought we want a distinction between treatment and enhancement for such purposes as figuring out what the task of the physician as such is and what requests from the patient the physician has a right to turn down flat. For instance, a physician who receives a request to remove a cancer, and who judges that removal of the cancer is feasible, safe and ethically permissible, has a medical duty to either remove the cancer or refer to someone else. On the other hand, a physician who receives a request to pierce a patient's ears for earrings, even though she no doubt judges this to be feasible and ethically permissible, has no medical duty to perform the procedure or refer to someone else, since it is not a medical treatment.
But a new kind of case seems to me to make the distinction even more pressing, and this is cases where it is not possible to ask the patient's consent. Suppose that in the middle of heart surgery, the surgeon notices an old bullet lodged near the heart. The bullet does not impair the heart's functioning, so the patient's consent to the heart operation does not extend to the bullet. But it is intrinsically morally permissible for the surgeon to remove the bullet if she reasonably judges that doing so is good for the patient (of course, there may be laws and regulations that prohibit this, in which case it will be extrinsically impermissible). On the other hand, if a brain surgeon removing a cancer from someone's brain reasonably judges, on the basis of the latest research, that moving a few neurons around will make the subject super-fast at arithmetic with large numbers, that is unacceptable. Likewise, if in the course of a Caesarian the physician notes that the tubes could be tied and judges that the patient would be better off not getting pregnant, that too is unacceptable, whether or not consensual sterilization is permissible (this is, alas, not a hypothetical case).
One can try to handle this with "presumed consent", but that's kludgy, and probably doesn't work. Presumed consent from an unconscious suicidal patient for emergency treatment following the attempted suicide is going to involve dubious counterfactuals, like asking what the patient would want if the patient were fully sane (there might be no fact of the matter about this), and, besides, you probably can't make sense of "sane" without the concept of normalcy. Moreover, we can imagine cases where one can presume that the patient would consent if asked, but the action is still wrong. For instance, one may well know of many patients that they would agree to have a gift of diamonds worth millions sewed into them as a part of surgery, if they were going to be later notified and could have the diamonds safely removed through another surgery and if there was no other way for them to be given the diamonds. But to sew in the diamonds as part of heart surgery, without having sought the patient's consent, is morally impermissible--or at least it's bad medicine.