With Johjima looking like he's on the verge of signing with Seattle (or New York, but whatever), it's probably time that we take a somewhat serious look at how we should expect him to perform in 2006.
To my knowledge, there have been seven high-profile Japanese position players who've signed Major League contracts - Ichiro, the Matsuis, Norihito Nakamura, Tadahito Iguchi, Tsuyoshi Shinjo, and So Taguchi. Three of them turned into good-to-great hitters, two of them decent role players, and two of them fairly spectacular busts. Of course, it doesn't really do us much good to compare Johjima to guys who weren't all that great to begin with (Taguchi, Shinjo), and we have too little information about Nakamura to reach any conclusions, so let's just go with the Ichiro/Matsui/Matsui/Iguchi foursome:
Last Season In Japan:
Ichiro: .387/.460/.539, 54:36 BB/K
H Matsui: .334/.461/.692, 114:104
K Matsui: .305/.365/.549, 55:124
Iguchi: .333/.394/.549, 47:90
First Season In Majors:
Ichiro: .350/.381/.457, 30:53
H Matsui: .287/.353/.435, 63:86
K Matsui: .272/.331/.396, 40:97
Iguchi: .278/.342/.438, 47:114
On average, the group saw a 12.7% decrease in BA, a 16.3% decrease in OBP, a 25.9% decrease in SLG, and a 32.6% decrease in BB/K in their first seasons in the US. Thus, Johjima's .309/.381/.557 2005 batting line translates into a .270/.319/.413 performance in 2006. Nothing fantastic, but certainly light years ahead of what we had last season. (Based on the data, it's hard to say what Johjima would do in 2007 and beyond, given that Iguchi's only had one year in the big leagues so far, and Hideki Matsui's improvement is negated by Kaz Matsui's suckitude.)
While it's tempting to say that Johjima's strike zone discipline (he's posted BB/K ratios around 1 during his peak) would help him make a softer landing, that hasn't demonstrably been the case with the players we've already seen make the transition - Ichiro and Hideki Matsui have seen just as much of a performance decline as Kaz Matsui and Iguchi, and only look to be better players because they had considerably higher statistical baselines in Japan.
If PECOTA has shown us anything, it's that historical comps can only go so far when it comes to projecting the future. That said, Johjima isn't as good a hitter as Ichiro or Hideki Matsui, and their numbers took a big hit when they crossed the Pacific. If and when Johjima finally lands in Seattle, it would be unwise to pile Varitekian expectations on his shoulders, but guys like Damian Miller and Mike Lieberthal are still good catchers who help teams win games. At something in the neighborhood of $4m a season, Johjima's probably going to be one of the better bargains of the winter. Full steam ahead.