I am from Chicago and while not a huge Cubs fan, I can't help but be encouraged by the current team. At 103 games, they have a run differential of +163. While its hard to project the future due to injuries, etc., with the closer they got from the Yankees, it would seem like they would have a good chance of increasing that number, possibly over 200.
Where does that rank in modern baseball?
Seven teams had a season with a run differential of 2.0 runs per game or higher, none since 1939. Going in reverse chronological order, they are:
1939 New York Yankees, 2.7
1937 New York Yankees 2.0
1936 New York Yankees 2.2
1931 New York Yankees 2.0
1927 New York Yankees 2.4
1906 Chicago Cubs 2.1
1902 Pittsburgh Pirates 2.4
Some time ago the Cubs were on pace to join this group, but their pace has slowed. Through their first 59 games (41-18), they were averaging 2.5, which would have been #2 of all time, but have since fallen back to 1.6.
Among modern teams, the 1998 Yankees and the 2001 Mariners came close to the 2.0 barrier. They were both right around 1.9.