Theres a line to be drawn here and it depends on the functionality of the implant. Implants that restore normal function versus implants that do something else which healthy humans could not achieve without.
Yes, it is certainly useful to differentiate between normal ranged human properties, and those so far exceeding the norms we have zero evidence on what to expect.
(So far the evidence seems to point towards the notion that you can't increase human performance by pushing along a single axis. We are near a local optimum in a massive, multidimensional space. Pushing too much in any one direction will move us farther from the local optimum.)
We're facing this everyday with rich and powerful making likely bad choices, for themselves and for others. Why would any other kind of superhuman be different?
The question is only of magnitude - what could one person do? How would they do it? What would be the cost or benefit?
This doesn't directly imply that interventions that make someone "less human" should be avoided.
That's not a rationale, that's just a criterion. My question is, "What is the rationale for avoiding these types of changes?"
Where is that limit achieved? In the case of prosthetic limbs, should we seek to achieve perfect organic behavior replicability, or is anything further than that inhuman in some arcane way? Oscar Pistorius, AKA Blade Runner, earned Olympic medals despite having prosthetic legs. Is he advantaged or disadvantaged compared to other participants?
The anime series Texhnolyze touches heavily upon this subject, amongst others.