The cruise level in flight is not necessarily indicative of the single engine performance on departure. Even at light weight the 777-300ER cruises much lower than some more modern aircraft and even some older ones but it's still got excellent OEI climb at MTOW. Don't have first hand knowledge of the A350's performance but I'd bet a decent sum that's it's pretty good. And with RNP departures Mt Majura is not as big a deal as it was a few years back.