An approximate rule-of-thumb with single-based powders is pressure increases twice as much as the added powder. This means a 7.5% increase in powder would increase pressure from 56,500 psi to 65,000.
With double-based powders pressure increases quicker, but varies depending on the amount of nitroglycerin content. In general it would probably be three times as fast as any increase in the powder charge.
I've pushed the 9.3x62 to around 60,000 psi (measured by strain gauge) in a 23.6" barrel, resulting around 2650 fps with 250-grain bullets with RL-15 and Varget, and around 2500 fps with 286's using Ramshot Big Game. That seemed to be plenty so I quit there.
