You may check out his numerous publications about the aircraft guns, most notably the books
Flying Guns: Development of Aircraft Guns, Ammunition and Installations (three volumes: WW1; WW2; The Modern Era)
and
Rapid Fire: the Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces
Also note he isn't part of the "Ki-61 POS conspiracy". He didn't say anything about the Ki, he's publishing & researching about guns.
The performance of the Ho-5 is well known & documented, btw.