Two comments:
First the SKS's and AK-47 locking systems depend very much on rounds developing approximately the same pressure. If they don't, then the rifle won't function correctly. This is generally true of semiauto and full-auto rifle actions, and since pressure often varies both due to amibient temperature AND a heated-up barrel, it's better to have a lower pressure standard to insure correct function.
One general rule of internal ballistics is that muzzle velocity increases only about half as much as pressure, though that depends to a certain extent on whether the powder is single- based or double-based. As a result, increasing the pressure from 50,000 PSI to 60,000 won't generate a proportional increase in velocity.
