If your company's budget is limited to the 15% of the cream, then maybe the "Bell Curve Method" can be an effective tool for you. You can provide good benefits and incentives to employees whose Performance Rating falls "within the top 15 %" of all the employees
that were "forced fit" into the curve.
But you may need a good Performance Appraisal System (or PMS) to create a credible profile of the true 15% performers. Without a good tool, your company can be accused of subjectivism and favoritism. When this happens, you have created a problem that you never wanted to happen in your company.
If you need a good PMS, my company has one. It is a complete system that was benchmarked with the best practices in variouos industries, and applicable to any type of company in any part of the world.... But we sell it.