Salute
Read the account carefully.
Johnson says the turn was so hard, he grayed out, ie. it was at high speed. You don't gray out in a normal 300 kph turn in a Spit. In IL-2, in an initial turn from high speed, a FW-190A4 can follow a Spit V. The circle will be wider for the FW, but it will get around faster.
Then Johnson clearly states he went into a dive, and that the FW-190 followed him through the diving series of turns.
Ie. the turns continue at high speed, (because of the dive acceleration)
In IL-2 if you take an early 190A4, and a Spit V, have the Spit do a dive, followed by a series of turns at high speed, you will find the 190 can follow the Spit.
Johnson probably made the mistake of diving, because a 109 cannot follow a Spit through a series of diving turns. (109 turns worse at high speeds)
The other factor we can't be sure of, is the fuel status of the aircraft in question. Ie. was the FW-190 at 1/4 tank and the Spit nearly full? We don't know. The early 190A's had quite low wingloading comparative to later models, (approx. 41 lbs/sq/ft at full fuel load, compared to 48 lbs for later models) if it was lightly loaded, then it might have come closer to a Spit V in wingloading.
The Royal Aircraft Establishment did at least 5 systematic and exhaustive comparisons between various 190A's and Spitfires, and found all the Spitfires were able to outturn the 190 in normal horizontal turn at medium to low speeds.
Capt. Eric Brown, the Royal Navy Test pilot who evaluated German aircraft wrote the following account of his test of a 190A4:
http://www.pbase.com/chrisdnt/eric_browns_190_report