Didn't the avgas used in these tests positively affect the performance of the Japanese aircraft?
It's a common myth that the TAIC performance numbers are derived from US tests with high octane fuel. In most cases, they're just calculated estimates using data that was often dubious. The ones that are derived from actual test data are much tamer.
Reasonably, they would rather overestimate than underestimate the enemy's new fighters at the time, which is pretty overtly stated in the TAIC manual.
Compared to Japanese performance test conditions, very high-octane fuel would certainly not hurt, but would be unlikely to significantly improve speed performance. That is, unless modifications were done to run the engine at higher performance than design, which is unlikely.