make much of a difference in MLB batting averages?
IMO, yes. 6 inches is about a 1% difference in reaction time. Sounds trivial but IMO, very significant.
Back in 1969, they lowered the mound from 15" to 10". In 1968, Bob Gibson had a 1.12 era and Denny McClain won 31 games. Not sure if there was a cause and effect.