Can you get to 25 in a zeke?
Yes, in particular the A6M3 and A6M5 both handle altitude quite nicely up to 20k, but are still reasonably usable at 25k, other than being slower than most any other fighter you'll meet up there. Certainly I would not want to be in an F4F, P-39 or P-40 engaged in a fight with an A6M3 or A6M5 at 25,000ft.