REGULAR Onigiri is low-mid-tier, I agree.
PURGATORY Onigiri, however, is easily mid-tier, possibly even mid-high given that he 1-shot Killer (low-commander lvl) with it.
King TANKED Purgatory Onigiri and got back up immedietly after, Queen exclaiming that they "didn't feel a thing"
Ultra-Tiger hunt on the other end broke through King's armor and probably did even more damage that we will see when the chapter drops.
Ultra Tiger Hunt > Purgatory Onigiri (mid-tier) = Ultra Tiger Hunt is High Tier
:holdthisl:and move on
Also Zoro used it against Fujitora in Stampede so yeah its a high-tier