You should have no reason to keep bad or average employees so everyone should have the same top-class salary. You might make exceptions depending how well your hires negotiate.
See what other local companies are paying and pay a little extra. Check this annually.
Avoid all ranking midst employees. Make sure that metrics you use don't cause competition. Good work should be rewarded, but not to shadow another employee.
At Valve, everybody has exactly same basic salary, but you can get 500% - 1000%
bonuses of that basic salary. Bonuses are given through peer reviews.