Washington Post has an article about moving the pitching rubber back 1 foot in the Atlantic League, which happened one week ago:
https://www.washingtonpost.com...-move-mound-changes/
Basically, they say the results were minimal:
As of Thursday, in 67 games played from 61 feet 6 inches, teams are combining for 6.37 runs per game, according to data tracked by MLB. Before the switch, they combined for 6.33. The strikeout rates MLB is hoping to suppress actually have increased somewhat in that tiny sample, from 18.3 percent to 18.8 percent. The walk rate has decreased from 12.4 percent to 11.3 percent.
But the pitchers are more sore after pitching the bigger distance. So maybe over time the effects will be greater.