A 10x developer is not ten times more productive than the average developer, they're 10x more productive than the worst developer. Wish this myth would stop perpetuating, they're very clear in the original study. Here's some more details from the horses mouth, Steve McConnell who popularized the concept in Code Complete: http://www.construx.com/10x_Software_Development/Origins_of_...
I've always hated this term and the mindset around it. I think organizational practices, intelligent engineering strategy, etc are far more important to the output of a team than hiring one genius dev.