He's 25, so he's probably not going to add anything to that fastball velocity, and probably isn't going to get too much better, though he's still young enough to sign a contract where you're not paying for his decline phase. Wikipedia has him averaging about 90 on the fastball, so his changeup is really key unless he either locates the fastball really well, has good movement, or has some deception in his motion. He hasn't walked very many (2.7 bb/9), but obviously MLB players are likely to have much better pitch recognition and be much more selective than KBO players. How does the KBO's level of competition compare to MLB or perhaps NPB?
It's been described as roughly equivalent to AA ball, but those equivalencies aren't very useful IMO, because of the obvious fact that if a player dominates AA, he gets moved up to AAA, whereas if a player dominates the KBO, NPB, or Cuban Serie Nacional, there's nowhere else for him to go. So while the talent level at AA is relatively constant, in leagues like the KBO it's much more variable. I mentioned this last year when Cespedes defected. Skeptics pointed out that the Cuban league is regarded as A+ or AA. While it might be true that most of the players in Cuba couldn't hack it above AA, it turned out that Cespedes was one of the best players in the world.
It's the same situation in Korea. The league has players who wouldn't rise above A ball here, but others who could be amongst the best in MLB. Evaluation of players in these foreign leagues really has to come down to scouting rather than statistical analysis.