Many years ago, when almost every site only allowed 328fps on a .20, there was a massive lack of knowledge on FPS with heavier ammo/joule creep, & all the other variables that can effect power in Airsoft, add to that the insurance companies knew even less so generally rather than investigate & attempt to learn more, they took the lazy route & just refused to cover anything other than 6mm use, most sites also wanted an easy life & accepted this, after all it's not like there's much about in 8mm ?.
I'm sure some of the brainboxes on here can work out an accurate FPS/joule reading with the relevant info, personally I'm with @Rogerborg on this, if it chronos below about 330, say nothing & play on, no harm no foul 🤔