I think he's going to be great for the first two years. People will have trouble figuring him out early on, and he'll have a couple of stellar seasons and a ROTY/Cy Young candidate.
Then when people begin to figure him out, he'll probably have to adjust his delivery and will end up just being a solid mid-rotation starter. The first couple of years, he'll be worth his contract, but after that he'll become Randy "Woof" Wolf with too long of a contract.
Not that I don't like Randy Wolf... he's certainly been a better investment than Barry Zito. But if I were running a team, I wouldn't have placed that much money, or that many years, on someone who has never pitched in the MLB.
I mean, it's hard enough to calculate "MLE" but with entirely different leagues and a much smaller sample size of players (Japanese pitchers in MLB) you really can't decide that there is a set trend for them to follow.
Of course, this isn't just about giving an unknown commodity a 6-year contract. Pitchers in general shouldn't be getting really long contracts because of the risks of injury and BABIP fluke seasons that can fool those who aren't careful.