Synthetic parts are commonplace already.
Teeth (fillings and false)
Just a few, but none of those improve on the original design they just replace the bits that have failed.
Now actual improvements are harder to quantify do you look at the blades amputee runners use and say are they better than the original limbs. What about if someone comes up with a Star Wars type artificial hand should it perform as the original or should it be stronger?
That is the sort of thing when the technology is working that people will have to decide for themselves.
I am very colour blind do I want the ability to see a normal colour range - well having lived in the world I perceive for 47 years I don’t honestly know, unless I was going blind then probably not, but different people will have their own views.