The High octane fuel is not needed.
AH Ki-84-Ia (aka Ki-84-Ko) using IJAAF's mass production trial model data, 624km/h at 5000m.
The engine was HA-45-Toku(1800HP/2900RPM/+400mmHg), The HA-45-Toku is power limited version of HA-45-21(2000HP/3000RPM/+500mmHg).
Also this trial model equipped normal exhaust manifold. (the mass production model changed to augment exhaust pipes. it improves speed to 10~20km/h.)
In fact, according to Japanese reference, the mass production Ki-84-I-Ko's max speed was up to 650~655km/h with HA-45-21 2000HP engine and augment exhaust pipes. Also Ki-84-Otsu (4x20mm ver) recorded 660km/h at 6000m.
These all data have been recorded when engine condition was "good".
IJAAF has limited HA-45-21's power to the same as HA-45-Toku in the war.
But Aces High simulates engine condition is always "perfect", so Ki-84 can be gotten more speed.
Please adjust the Ki-84.