And here I was... Thinking you were. I have not fought in an A6M at 20k altitude, but from what I can tell at 5k alt, if the F4F pilot is any smart, a simple dive would let him extend and come back with a better position, should the Zero continues to chase, he'll either rip off his wings or have to throttle down, in which case, the F4F could use his speed advantage, zoom up, loop and blast the slow Zero.
Against other planes, even an La7 who loses much past 12K, would still be able to outfly a Zeke. IMO, the only way a Zeke is able to survive in the MA is to sucker someone into a TnB or against other early war planes it can chase or keep up with. I also can't imagine why a Zeke would be at 20K, it can't dive long without turning into a flying fuselage.
