Apologies everyone. Marzimallow's Tempering Master was a bit too powerful coming out of the gate.
With version 287.1. we've made some changes to them.
The base of Tempering Master has been reduced from 15% to 5%, and the legendary effect from the Legendary Bunny gear item has been reduced from 300% to 100% additive per level. They're still powerful, just not as overpowered!