Ye. crafting mats should stack effect so x0.2 + x0.3 breast should give x0.5 and so on, but seems that i bugged them.
For mats balance i plan to properly rebalance them, as for now it was mostly "I just throw random numbers at it without more thought"
There will be better explanation and for example Damage deviation split into Max & Minimum stats.
For now bigger enemies doesn't not increase drop rate.