F4UDOA, your mistaken in this case.
It was the Ki84 that was tested with 100 octane fuel postwar, and made 427mph.
DarkGlam, notice that Butch's chart says TAIC?
These aircraft performance and characteristic were issued by the Naval Intelligence Division,US and British Services attached to the
Technical Air Intelligence Center , Naval Air Station, Anacostia DC
Note from the TAIC manuals:
Performance Calculations: Except where otherwise noted, performance figures represent estimates of the Technical Air Intelligence Center adn have been calculated after careful analysis of information derived from intelligence, captured equipment, drawings, and photographs. When authoritative evidence was not available, it was the policy of the TAIC to give Japanese Aircraft Performance every benefit of the doubt within reasonable limits.
So basically these were estimates and calcualtions of enemy aircraft performance during the war. However, in reality the Japanese airforce and naval air forces suffered from low production quality and lack of high quality av gas which greatly decreased effectiveness. Plus, there was a "safety margin" built into the estimates.
I'm not saying the data HTC uses is perfect, I'm sure its got some holes in it, but its at least the AH plane is based upon wartime flight tests by the Japanese manufacturers.