I don't follow the reasoning. Even if you have a reliable prediction of performance, what prevents you from hiring some candidates who are exemplary and some who are just well-qualified? Or are we assuming that the best candidates would be given a higher-tier job for which they just meet the requirements?
>>titano+(OP)
It's a spherical cow conversation. The cows you hire can be placed in a smooth manifold of job difficulty and you have a predictable prediction of the job performance distribution.