I've just had a look at what game-tape I could find for Greene. My major concern would be the limited time he faces press coverage. This, and route running, are normally the biggest reasons WRs fail in the NFL. His route running seems ok, but he does seem to run an extraordinary number of 9-yard curls. I think he would be a solid WR but I don't think he has a huge amount of upside. If he's there in the later rounds (6 or 7) or as a UDFA then it is definitely worth a shot. But players like Mike Davis will likely still be there in the 4th.
Similar to Greene in many ways, just possesses more upside. Would work well as a slot receiver at the next level.
I think Greene upside is higher because of his speed and Yac ability. I do not think Davis is as fast or quick twitched as Greene. Greene does run more then curls, I have seen him score on post, outs, and slants numerous times. I do wonder how he will handle press man, because he hardly sees this coverage because of his deep speed. He can be a body catcher at times, but is very willing to go over the middle.