"Premium status" on Smashwords means that besides being for sale at http://www.smashwords.com/, a book will be listed in the Smashwords Premium Catalog. Acceptance into this catalog means Smashwords distributes the book to major online retailers such as Apple, Sony, and Diesel eBook Store (Smashwords also used to distribute to Borders and the Borders' ereader company, Kobo). These retailers require that the book conform to requirements, such as a quality cover image, a copyright notice at the beginning of the book, and an ISBN (which you can get free from Smashwords).
I've been helping a few other writers, notable Destiny Drake, format their books for Smashwords. Destiny's books have all achieved premium status. However, Destiny says that although 16% of her total sales are coming from Smashwords, not a single sale has come from outside http://www.smashwords.com/. In other words, no sales from Apple, Sony, Diesel eBook Store, etc.
To me this says, "Don't waste your time trying to conform to the rigorous formatting requirements of the Smashwords Premium Catalog." And especially, don't waste your money paying someone else to do the formatting, unless you're just a perfectionist, and unless you're rich, and unless you're confident you will sell to the "major online retailers."