I guess that the estimated max. speed of 460-470 km/h at the altitude of 3500-6000 meters refers to brandnew Ki-27b (Otsu) without full load. And most likely a very optimistic prototype figure without ammo load, not having full fuel load, paint etc. A comparison could be made with Curtiss P-36/Hawk 75 which practically never made it up to overoptimistic Curtiss factory figure 500 km/h which is often referred (without criticism) in English/American literature. More of this at another forum:
http://forum.warthunder.com/index.ph...6#entry3817707
The combat records of Ki-27 pilots over Burma in the spring of 1942 state that they had difficulty to catch Blenheim Mk IV (according to one Japanese pilot it was actually faster than their "Type 97"). My rough guess is that the well-used airframe (the Ki-27 were already quite old planes in 1942) with combat load could not be more than 420 km/h at its optimal altitude (probably 4000 - 6000 meters).
I have read that AVG pilots made extensive tests with one captured and intact Ki-27 Otsu in the spring 1942 but I do not know if their test results have been written down and published anywhere. Also Claire Chennault testflew one Ki-27 and supposedly wrote a report of its performance for the US military intelligence.