Generally, over the years, I haven't seen a hitting coach make much of an impact on the overall production of a major league team. There are exceptions, as some have acquired notoriety (Lau, for example), yet even the "best" have had there detractors. You also get a situation where a hitting can make an impact on 1 or 2 players, as something just clicks between them. But, all and all, hitting coaches come and go and nothing much changes.