The OP talks about 'ability' which rules out things like 'Opportunity' or Injury Tolerance. This argument is much more interesting if we accept that genetics is the #1 factor, forget about the external stuff and concentrate on what parts of 'genetics' factor in, and how much. Anatomy, Physiology, Training Response, Psychology (brain chemistry) etc. etc. This is almost impossibly complex, which is why we still can't predict performance and, after all these decades of research we still don't know why we have good/bad days!