See below, a screenshot of someone looting two scarab cores when scarab was the daily boosted creature.
Here is another loot message, this one is not a screenshot:
11:43:16 Loot of a sopping carcass: 11 slivers, 2 sopping carcass soul cores, a crystal coin (Bakragore's tribute).
As you can see, at least these two types of loot boost work, so we can be quite confident that all loot boosts will affect it.
Note that there are other rare drops from fiendish creatures: they can drop soul prisms, and very rarely, they will drop a second soul core of an uncommon, rare, or very rare creature within the same bestiary class. Whether these drops are also boostable has not been confirmed.
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