I definitely depends on how the Sox spend and where he plays. If he's at 2b, I think the position will be better off overall due to his defense. At 3b, there will be a dropoff. I'd assume there'd be a big signing for 1b in that case though.
Yeah, I'm sure every team that has ever coached him has tried to work with him on not striking out so much. Just saying "here's some ways to do it as you transition to the hardest league in the world" is a bit of a stretch. If he k'd about 30% in NPB, he's going to k about 35% in MLB. For qualified batters, Ryan McMahon led MLB at 32.3% last season. His strike out comp is probably Patrick Wisdom who is now playing for the Kia Tigers in Korea.
He's a 2/3 WAR guy today. The problem is that he won't be in a few years. The problem is the sheer length of the deal, not what he's currently being paid.
That's 12.5M for 8 years! Those last 3 are a write off at least. He's going to have to be a 2.5 fWAR guy every year for the next 5 years for even 50% of the contract to work out. Not a given!