The AS boost is amazing already. Miss Fortune is a pure-damage carry and she gets a smaller boost than that!
But yeah, that build looks solid. I'd probably go with Sorcs instead and finish Morello's ASAP, but it's up to taste.
--
Currently playing: League of Legends, Chrono Cross.
Current Projects: Fool's Overture, CYOA: http://www.gamefaqs.com/boards/8-gamefaqs-contests/59835001