All people operate within a system, whether visible or not. Improve your systems, improve your quality; regardless of your employees.

Besides, by definition, most of your employees will be average. Otherwise the average would shift. Focus on bringing the whole bell curve up by improving your systems and your management, rather than on terms you use for individuals. You're better off optimizing your company and your management to handle this diversity, since it's a natural result of a sample.

You are assuming people who are 'high achievers' achieve highly in all circumstances: this is false.

For example, the Ivy League colleges suffer rampant grade inflation due to these high-achieving students not being able to cope with getting less-than-perfect grades. This poor coping is a form of 'low achievement' that drives other 'low achievement' behaviors: reduced course load, dropping out, switching to less rigorous majors, etc.