I don't use PO in the infield. That's largely someone throwing the ball to you. If a guy makes a great diving catch on a line drive, it wil show up as a + play. The story it tells, so to speak, is who is fielding the position well and which position is making more isolated plays.
tec, the only one I'd really dispute is CF/3B(and it's really the only one that's close). Of course, I lean a little bit more defensively towards CF and care a little less about GB/FB than most(except in Scottsdale).