Firstly, no, a 50 Mph speed advantage WILL NOT give you enough vertical separation to survive a zoom versus a great accellerating plane like the N1k2. IMHO. I would expect to get killed trying that against a G10 or a Spit IX as well.
Second, I doubt the N1k in question really "completed his turn". More likely he was watching you and guessed what you would do. (Not tough, what else would you do?) When you went vertical I'd bet he simply pulled up and rolled onto your flight path. A great accellerator like the N1k could do this and still have enough power to hang on it's prop and gun you from the sky. As it seems he did. I'd want about 100 mph advantage to try that versus a N1k or a G10. Better to use your speed to extend a bit for horizontal separation, THEN reverse high and try another pass. You didn't have enough E advantage to get the vertical separation you need to stay out of guns range. IMHO of course.
BTW, remember that on his FE that D 600 shot was likely more like 450. Not a particularly tough shot versus a nearly stopped target.
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Lephturn - Chief Trainer
A member of The Flying Pigs
http://www.flyingpigs.com 
"My P-47 is a pretty good ship, she took a round coming 'cross the Channel last trip.
Just thinking 'bout my baby and lettin' her rip, always got me through so far."
- Steve Earl
