According to 82games.com, Jeff Green is better as an SF than as a PF:
In 09-10 as a PF, Green had a 16.7 PER, his opponents a 20.1 for a difference of -3.4
In 10-11 as a PF, Green had a 13.3 PER, his opponents a 21.5 for a difference of -8.2
In 09-10 as a SF, Green had a 10.5 PER, his opponents a 11.0 for a difference of -0.5
In 10-11 as a SF, Green had a 11.0 PER, his opponents a 13.2 for a difference of -2.3
I guess his offense suffers when he moves to SF because he doesn't have the quickness advantage he has over PFs. But his defense improves a lot, so much so that opponents produce below average. Of course, it's disconcerting that Green is below average as well...