Player's power numbers go up once they're in the Major Leagues all the time. I've been looking through players with builds similar to Drew Stubbs' (tall, lanky), and almost all of them saw nearly a 50% increase or more in their homerun production.
Through 2+ seasons in the minors, Stubbs averages a homerun every 54 plate appearances or so. Shannon Stewart has put together a heck of a career doing that in the Major Leagues. The thing is, if Stubbs becomes a regular in MLB, he will play better defense, steal more bases, get on base just as much, and almost definitely have more power than Shannon Stewart.