Nothing remotely close to data crunching here, just observations over the years (so, obviously, there will be mistakes and omissions).
The art of predicting can be tricky. Nobody knows exactly how to predict the future. I was curious how some Japanese pitchers would project to MLB so I created an index based upon past Japanese pitcher performances.
http://www.japanesebaseball.com/forum/thread.gsp?forum=21&thread=59243
There’s of course the big x-factor, the cultural and environmental and mental adaptation process that players crossing the Pacific (in both directions) have to endure. Some thrive (Nomo, Okajima, Saito, etc. with the latter two outperforming their NPB records), some fail (Kobayashi, Fukumori, Igawa, etc.), some fluctuate (Irabu, Matsuzaka, etc.), some thrive then fade (Sasaki, Takatsu, etc. though this might be due to more age and wear and tear) and others meander (a little early to tell but Kawakami and Uehara seem to be adapting well). I don’t think there’s a large enough sample size yet for reliable equivalencies when it comes to pitchers.
Position players, on the other hand, are more predictable creatures, so their production are more predictable when making the NPB=>MLB move. BA/OBP drops somewhat, HR gets cut in half (and SLG drops accordingly), defence is usually above par (except whatever happened to Kaz Matsui), and many hitters change their approach at the plate (Godzilla not swinging for the fences, Iwamura exchanging power for on base ability, Little Matsui exchanging power for speed, Ichiro becoming more or less a slap hitter, etc.)
So, predicting Darvish? Well, he pitches half his games in the most pitcher friendly park in Japan (Sapporo Dome) in front of a solid defence, so right there are strikes one and two against him if he makes the move across the pond, unless he ends up in Petco with the Pads, Safeco with the Mariners (only possibility?), or at the Coliseum with the A’s, since most modern and ancient MLB parks are of the hitter friendly variety.
So, as the usual caveat goes, pitching is much less predictable than hitting, on a season by season basis.
Just my rambling 2 yen late at night when I should be sleeping.
