The 1927 Yankees have the highest OPS differential since 1914. Their batters had an OPS of .872 while their pitching staff allowed .676. That gives them a differential of .196. See my post on The 25 Highest And Lowest Team OPS Differentials From 1914-2014.
Before 1914, we don't know SLG allowed by teams. The Cubs had an OPS of .667 and allowed an OBP of .280. With SLG unknown, we can't get OPS.
But I can estimate it using their AVG allowed (.207) and HR% allowed (.00244 ). I ran a regression using all the MLB teams from 1914-1919 with SLG allowed being dependent on AVG allowed and HR% allowed. Here is the equation
SLG = 2.497*HR% + 1.257*AVG + 0.00264
That predicts that the Cubs would allow .269 and given them an OPS allowed of .549. Then their differential would be .118, far below that of the 1927 Yankees.
Now the team that fell the farthest below their prediction for 1914-19 was the 1919 Giants (.012). That was equal to about 2 standard errors of the regression. Even if we lower the SLG allowed by the Cubs by .012, their OPS differential is .130, still far below that of the 1927 Yankees.