I decided to check the value in the field <SyphonManaCoef>0.5</SyphonManaCoef>
And what turned out? It doesn't affect anything (or I'm not understanding something).
Initially, I thought this applied exclusively to Pit Fiends, since in the game files their ability is called SyphonMana. But the test showed that Imps/Familiars and Pit Fiends drain the same amount of mana.
The test was conducted with the value in the field <ManaStealPerWeekPopulation> = 1. I'll attach screenshots below for clarity.
And here a question arises for me — shouldn't Pit Fiends drain less mana compared to Familiars? From memory, it seems they should.