Shonn Greene ran a 4.62 40 time, and that was at the NFL Combine. It's plain as day that 2012 Shonn is not nearly as fast as 2009 Shonn.
Powell ran a 4.54 at the Combine. He broke a bunch of very long runs in college. While he may not have elite NFL speed, he certainly provides way more speed than Greene, and he is (more) capable of taking one to the house.