I'm no AB driver, but the 315 limit in dependence of operable speed brakes makes sense from a rapid descent perspective should the other pack decide to quit also, which is far more likely, statistically speaking, when one is out already because the remaining one is working harder.
Some subsystems have pressure dependent components (we do...) hence have a limited amount of time to get down. Remember TOC is limited even on full face but non-pressurised oxygen masks from 350 and up, if memory serves.