What you're referring to is genetic potential which is:
...theoretical optimum performance capability which an individual could
achieve in a specific activity, after an ideal upbringing, nutrition
and training. In real terms it may be assumed that the finalists in a
world championship are among the human beings whose performance comes
closest to their genetic potential. Also known as genetic endowment.
See also performance genes.
I am unaware of any studies that conclude genetic potential exists as any formal study would take decades to do (even then may be impossible to do properly).
However, it is generally regarded that everyone has a certain performance limit they are capable of, and the closer you get to the point, the harder fitness becomes. Some people have very low genetic potential while some people have very high genetic potential.
There are multiple factors that could limit performance such as (but not limited to):
- Limited physical ability. For example, people unfortunate enough to be born with skeletal deformities would not be as capable as someone without such impairments.
- Limited ability to grow muscle.
- Limited ability to recover.
Beyond that there are people who are just physically better built for a certain sport which gives them a competitive advantage. For example, Michael Phelps is regarded by some as having the "perfect body" for swimming.
Beyond that you start getting in to even more theoretical limitations on what the human body is physically capable of. So far, to my knowledge, we have not found these limitations, but one must assume that they exist. The human spine can only hold so much weight, the legs can only move so fast, etc.
Beyond that you start getting in to what's physically possible. For example, if Superman existed, he wouldn't be able to pick up a plane because his hands would punch straight through it.