40000ft in an A320. Why would you want to? Optimum fuel economy altitude is way below 400 at normal airliner operating weights. The ACJ is much lighter towards the end of its mission or on shorter range sectors and thus can justify the higher altitude for some fuel benefit. Also the greater routing flexibility of 410 on shorter ops away from major routes brings a benefit for the ACJ. But an airliner, working on airways... it brings nowt.