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I definitely need to read more James. That kind of historical study is far more convincing to me, and thanks for slowing down to present your side more fully.
One quibble with the sample, however. It seems to eliminate teams for which the team leaders improved, but the rest of the team did not. If those teams outnumber the teams in James' study, that would suggest to me that we really haven't a reason for why sometimes the whole team improves and sometimes it does not.
If Ichiro, Figgins, and Smoak have monster years, for instance, but Jack Wilson, Michael Saunders, and Adam Moore flounder and the team struggles to stay around .500, is that not a counter-example to James' study?