I can climb to 7400-7600 m, full throttle, manual PP around 2300 engine rpm. Climb speed around 200, with slow (i mean very slow) climb rate. If you pull the stick, the wing angle of attack exceed the critical value, and it WILL descend, even if the nose is pointing upwards.
The ingame ceiling is 8000m (if the altmeter set to zero properly
, the Spit IIa stopped climbing in this alt.
There is a promise by the devs, that this will improve.