Didn't a lot of people think Derrick Williams couldn't play sf? I can see him playing both positions. I don't think he would be an all world defender at sf, but I think he could hold his own. Think of the matchup problems he would create at sf playing along side Favors and Kanter, but I could be wrong.
Williams is a good deal quicker than P.Jones. At 6'11 I just don't see Jones being able to move his feet fast enough to guard the perimeter.