500 fpm.
I just took a Spit IX to its limits. Silly me did it in the online MA, so the fuel burn was too high, - I ran out of fuel (full + slipper) before I hit the roof. But I was at 44K.
Those could cruise at 43K in 1942 (keep formation), - and already there a problem occurs that does not have place in AH, - "bends", - pain in joints due to the low air pressure. Sort of a diving sickness actually.
At 50K, a normal person cannot breathe anymore due to the pressure, that means that the oxygen needs to be pressed down your lungs, - inverted breathing, for you will need to push the air out.
(However, if you have strong oxygen, you can keep from fading for minutes).
So, 50K is basically the limit unless there is either a pressure cabin or pressure breathing.
Anytning over 40K on some basis will be possible, but very painful.