Matt, they use pitchers with different rest (5-6 days) in Japan, as I'm sure you know. My concern about bust potential is that he's significantly below the minimum height for a typical MLB starting pitcher, has never pitched on "normal" MLB rest, and shorter pitchers are thought to be more prone to arm issues. Even Pedro Martinez was taller than he is. If he were 6'1" or above I'd have much less concern.
I'd rather invest the money in pitcher who has shown he can handle a MLB innings load on traditional rest, or at least fits the physical profile of pitchers who survive the MLB workload.
It's a lot of money...a big bet on projecting success for a "unicorn" with very little supporting evidence of that kind of player succeeding. It's not my money, though...
Of course, if the team that signs him is right about him, with the big risk comes big rewards. It just feels like a long-shot bet to me.