Bullethead,
Actually that's not true. Japanese test data is available for some aircraft, and I believe the Ki.84 is one of them.
As to the quality issue. You Allied fliers don't have it easy either. If it was modeled correctly our A6Ms would keep on ticking while your fighters (P-40s particularly) would suffer high attrition from mechanical failings. Of course this is reversed later in the war. Only the first few batches of Ki.84s were anything like reliable and the N1K2s never were. The A6Ms were reliable throughout the war though.