In my article in the 2014 Hardball Times Annual, I proposed the "10- and 20- rule of thumb" for players transitioning from the NPB to the MLB. In short: We can expect to add 10 points to a pitcher's FIP- and 20 points to a hitter's wOBA+.
If we follow that rule here, we would expect Wada to transition to a 95 or 100 FIP- pitcher in the Majors. The NL average ERA in 2013 was 3.74, so he could conceivably pitch to the tune of a 3.50 ERA, maybe better if he has some sort of skill with regards to beating his FIP (as it appears he does in the above selection of data).
BUT: Wada is not 31, and he is not the picture of health. He is in fact 33 and looking to regain his consistency after suffering TJ surgery in 2012. In 2013, pitching for the Orioles Triple-A affiliate, he posted an uninspiring 4.03 ERA in 102.2 IP.
When Wada has been on this spring, he's looked very sharp. When he's been off, well his 11.12 ERA tells the story. More troubling is his lack of control. He had a 23% K-rate and 7% BB-rate in Japan, but has had a 21% K-rate (not bad) and 15% BB-rate (super bad) this spring. Rate statistics like those do tend to stabilize quickly, but 34 batters is certainly not enough to definitively say one way or the other.
But he does appear to have a fairly filthy changeup, capable of stymying opposite-handed hitters:
Jake Arrieta (when healthy) or Chris Rusin long term, but for the short term, Wada has a nifty little upside.