I don't necessarily agree that medical applications impede function. I believe the majority of operations involving implanted 'technological devices' are purely functional, and typically consist of replacement of failing or destroyed natural body parts. Pacemakers, hearing aids, and artificial limbs and joints are all devices that aid people in retaining as much of their original mobility and functionality as possible. Typically current technology does not allow for these replacements to surpass (or often even rival) their natural counterparts. However, the alternative to these devices would often be complete loss of functionality, so these could be considered enhancements. In which case, the answer to your first question would be "when you need it."

The gap between natural and artificial will continue to narrow with improvements to technologies such as artificial sight ( www.temple.edu/ispr/examp...09_16b.html ) and tighter more functional integration of prosthetic limbs ( timesofindia.indiatimes.com/arti...0.cms ). The fact that people with prosthetic limbs are able to compete in sports on a professional level is obvious evidence that 'replacement' technology has greatly improved, and should be a good indication that it will continue to improve, hopefully at an increasingly rapid pace.

I believe that 'replacement' technologies will need to get to a point where prosthetics are on par with natural body parts before research will begin to move effectively in the direction of enhancement. At least it would need to for more practical applications such as visual zooming, visual information overlays and real-time sensory recording and storage. There may be more novel applications available before we reach that level, probably something gimmicky like a flash drive embedded in your hand that you can access via a wireless technology or the magnetic field generated by your body (There was an article regarding using this field as a vessel for data transfer, but I can't seem to locate it just now). So it would seem to me that the enormous volumes of effort and funding being poured into research in fields related to prosthetics and more generalized fields like biochem indicate that we will eventually get to a point where human enhancement is a reality, but I don't expect much within the next decade or two.