Ways To Buff Energy Drain For Monsters?

Energy drain is incredibly dangerous and honestly kind of cool, but I feel as though the monsters should get a bigger buff than a measly 5 temporary hit points per level drained. Any suggestions on how to increase their power in proportion?

Perhaps I should follow some of the monsters from BoVD and allow it to heal them as well, perhaps a few temporary stat bonuses?

Also, please don't assume I am cruel...