Hello, quick comment. I am both a retired military / commercial pilot (40 years flying) and an air combat simulation designer (tabletop, not computer). Also, I have studied WW2 air combat history as a passionate enthusiast for almost all of my adult life. I am of the opinion that it is best to not put much faith in anecdotal pilot combat stories that appear in books for a number of reasons - and, rather, to stick to the evidence of raw physics and aerodynamic design. With regards to anecdotes in books, such as Johnson's story... remember, that (a), the author and author's publisher want to sell the book to audience and exciting stories do that better. So there is the possibility of some exaggeration, perhaps not by the author, but by the publisher editor etc. Second, nobody can ever provide us the exact circumstances of that particular engagement - by which I mean, starting speeds, energy state, position and orientation of the two opponents at the actual start of their fight, so as to properly analyze it after the fact. Finally, I might add, that such anecdotes are built from the perspective of the pilot telling the story, and it is very possible that 1) he had the illusion of being out-turned due to the relative motion of the Fw 190 across his field of view, or that 2) he actually was being out-turned at that particular moment, not due to any built in superiority of the Fw 190 in turning, but due to the fact that their energy states were very different (he on the verge of a stall and the 190 still with energy which allowed the 190 to generate a greater rate of turn, despite having a wider turn radius from that speed). I personally have experienced both situations in mock air combats during my military days. The fact remains, regardless, that given equal energy states at the start of a turning contest - A Spitfire will always out turn an Fw 190 in the level.
Have a great day - Loved Johnson's book by the way.
J.D. Webster