You can setup different affiliate payout scales in several ways:
You can manually add users to an exiting program and change their payout to a higher percent. You need to go to the Programs admin page, select that program, and click on the edit icon. The bottom section is where you can edit the payout of the program. In the boxes below the line starting with default, enter the login name of the user whose payout you'd like to increase and change the payout to the desired one. Click save. Clicking the delete icon removes this special payout and sets payouts back to the default.
Or you can create a program with the desired payout in disabled programs, never enable it, and manually add affiliates to it.
Or you can offer this increase to all webmasters but only if they bring you a certain amount of traffic by setting up a Referral Payout Tier.