When new powersellers go onto the powerseller forum one of the first questions they ask is.
Do I pay less fees because I am a powerseller? The simple answer is no, however if you have over 1500 pounds (or the equivalent in dollars) going into your paypal account each month you can reduce your paypal fees, it is just well hidden on the paypal page, let me tell you were it is (unless they move it)
Log into paypal, select from my account, profile.
At the bottom of the page click on fees,
Click on the link in the table on the column receive funds/premier business acount.
Find the text How is Eligibility for Merchant Rates determined and click on Merchant Rate Criteria and at last you are there.
It takes a month for the paypal fee to kick in and only if your paypal income is high enough
Why this is not done automatically I feel is more down to business profits than anything else.