They only carried limited O2 for pax supply under JAR-OPS - that's why it's artificially limited. If they went higher they'd have to carry O2 for each seat, below 250 they don't have to.
I know of ferry flights where they've managed to step-climb it to nearer 300, that said, you'll be lucky to get it above that. In the summer we used to struggle getting it above 190!!!