I hadn't realised that the forum toilet attendant was also now an aerodynamics, performance, and certification expert.
Many -800 pilots like to be within 1000ft of optimum altitude; few like to be at FMC-stated ceiling.
That said, the aircraft and its FMC meet the relevant certification standards in this regard, with any FMC update. Having had an upset at 410 in another type, well within its margins, I suspect that insufficiently-demanding standards and/or greater-than-predicted wind shifts are the genuine issue here.
