Personally, I don't like the move, simply because McKay's two-way talents realistically don't boost his overall value. You've got people who feel he's a pitcher moving forward, but in this case is he even better than Kyle Wright from Vanderbilt? I don't think so, and in fact most people will probably tell you that they like Wright better. Then you've got people who feel his future is at 1B, but again even though he projects well here, you're now talking about drafting a 1B #1 overall...and how much better (if at all) is he than someone like Pavin Smith in this outcome?
With Greene you are at least getting the highest upside player in the draft, even though the risk is obviously higher. With McKay I just see too much of a chance wherein you make him a pitcher and yet Kyle Wright or someone else becomes better, or you make him a hitter and he simply only becomes "good" because the offensive expectation for 1B is so high.