you sure of that? NATS don't agree! Altimeters are only +/-200'.
You mean the regulatory max vertical navigation performance, right? Because my 737 tells me its actual vertical navigation performance is around 150 to 180 ft at FL410 and getting better (smaller) below that. Required vertical navigation performance is set as fixed 400ft throughout the usable altitude band and can, unlike lateral RNP, not be changed by pilot input.
However for my company deviating by more than 150 ft is an altitude bust that has to be reported.