This is now the 7th best offensive season in Chicago Cubs history with 8.7 fWAR, and the 5th (1958 Ernie) and 6th (1964 Santo) are both also 8.7. After that there's a small gap to 4th (1967 Santo, 9.5), 3rd (1959 Banks, 9.7) and 2nd (2001 Sosa, 9.9), and then an even bigger gap to 1st (1929 Hornsby, .380/.459/.679, 11.0).
On the pitching side, you've got 4 seasons pre-1900 that are measured at over 10 fWAR (all with at least 500 IP), and then 1970 and 1971 Fergie at 9.6 and 9.5, which seems within reach.
Obviously not a true counting stat, can certainly have a rough finish and move backwards, but likely looking at somewhere between the 2nd and 5th best season in post-1900 Cubs history by the time this wraps up.