Affects stat bonuses from passive items, trinkets that are absorbed or in a second trinket slot, and permanent bonuses given by items such as Void.
The extra stat upgrades are 33% of any final stat calculations over its base value (which is equivalent to Isaac's base stats), for example with 4.5 damage, it is 1 damage over the base 3.5, and the 1 damage gets buffed to 1.33, ending up with a total of 4.83 damage.
The extra stat upgrades are 20% of any final stat calculations over its base value (which is equivalent to Isaac's base stats), for example with 4.5 damage, it is 1 damage over the base 3.5, and the 1 damage gets buffed to 1.2, ending up with a total of 4.7 damage.
Fire rate multipliers change what is considered to be the base value, so with Soy Milk, only tears ups above 15.00 will gain a bonus.
The only exception is Eye Drops, which gains a bonus to its 1.2x fire rate multiplier if it's above base.
Direct fire rate modifiers like Mom's Perfume do not get a bonus.
Notes
The trinket does nothing for the listed boostable stats if they are at or below the base value; the boost is calculated on the total bonus, not per item.