Take away a couple of assists and add a couple of points, and you have Green's estimated production if he can learn to finish. If he gets 17 points and three assists instead of 15 and five, I don't think that makes a difference. Also, if someone is just a half a point/assist/etc. away from reaching a mark, they really should be counted. As far as I'm concerned, Andre Iguadola has averaged 15/6/5/2/1 per 36 over his entire career.
A ceiling is not supposed to be incredibly realistic; it's not supposed to be what the player will achieve if he barely improves. Maybe Green's ceiling is a less-athletic but better-shooting Iguadola. Outside of assists, their per 36 numbers are pretty much the same.