I think players often attribute not being about to out turn X plane because of FM changes, when in reality, it had everything to do with a specific encounter.
The Spits you out turned before, what fuel load did they have, E states, skill of the pilots, combat trim on or off? How do you know?
Too often the 1st thing folks say is "the FM changed" < and don't think of all the other reasons it may be.
Btw I killed a Spit 16 in my 51D last night, coalt merge at 15k, down to the deck, in a stall fite. So, it can still happen, but that doesnt "prove" a FM change because this or that happened to you. Sometimes a player stalls a Spit, sometimes not, every fight is different. Some cant fly N1K2s worth beans.
...Personally I find the 51s able turn very well, as long as they arent carrying a gazzilion gallons of fuel, and you use combat flaps where needed.