Thank you.
In that case I have also found no mention whatsoever of a VnE due to this or reports of detaching surfaces through poor manufacture or materials in the literature (as is the case with the poor heat treatment of landing gear, say). I have found one snippet which makes the AH random detachment 'feature' look a bit odd and random:-
"Flight trials confirmed the excellent performance and flying qualities of the new fighter but also revealed a lot of its imperfections. First of all, some characteristics - although better than those of the aircraft's predecessors - were far from what the Koko Hombu was expecting. The Ki-84 attained a maximum speed of 624 km/h at 6500 m, a ceiling of 12,400 m, and climbed to 5000 m in 6 minutes 26 seconds. One of the pre-production machines (no. 04) with a take-off weight of 3794 kg piloted by a Doi-kin Rikugun Kokusho pilot. Shoi Shiro Funahashi. managed to attain a speed of 631 km/h at an altitude of 6120 m, climbing to 5000 m in 5 minutes 54 seconds, but that was all it was capable of.
When pulled into a dive, the fighter accelerated to a speed of 798 km/h. but oxygen system trouble forced the pilot to interrupt this trial. In September 1943. during trials at the Koku Shinsobu, the Ki-84 was confronted in a combat simulation with the the Ki-44 and a German Focke-Wulf Fw 190A-5 brought to Japan in the summer of that year onboard a submarine. At the stick of the Ki-84 was another excellent IJAAF pilot. Shoso Yasuhiko Kuroe ace with 30 victories to his score — all victories according to [13]). The simulation confirmed the Ki-84's much greater agility than its opponents, although it also revealed that it was somewhat inferior to the German aircraft in terms of maximum speed and, more importantly, general behavior in the dive." - Leszek A. Wieliczko, Nakajima Ki-84 Hayate.
798 km/h = 496 mph and they we clearly not afraid to push it further. But in my experience in Aces High one (random) set of your control surfaces detach if you exceed 480 mph and above 450 mph if you make anything but the most gentle of inputs.