Re: "testing the limits of what's possible"
There are hearing aids implanted in the ear. They're called cochlear implants, and have been around for a couple of decades now.
There have been several experimental eye implants as well, I remember reading about one guy who went from total blindness to monochrom vision at a resolution of something quite a lot less than VGA. But I've not seen much about it since. Maybe they're still trying to find enough Matrox Millenium cards on eBay?
In principal I could imagine an implant that would pump glycogen directly into your muscles - which wouldn't give you more strength - but would give you more endurance - so you could say sprint over longer distances.
But the standard medical implants are things like insulin pumps, pacemakers and the like.
Though in the more science fiction realms - some artificial limbs are now radio operated. Because the nerves in the amputation site tend to be damaged as well - it's much harder to make a standard operation that utilises them. So instead you chop some nerves that don't do much around the side of the ribs and implant a radio transmitter that they can control. Then do excercises to retrain the brain to use those to control arm muscles instaead, then have a radio controlled robot arm.
Or there's a Parkinson's Disease thing where you have electrodes into the relevant bits of the brain. Feed current in from an external battery pack and hey presto! Full muscle control. Watching a guy on telly switch that box on and off again 15 years ago totally blew my mind.