Yeah, the drain does seem to be by far the most powerful aspect of his powers. Lemme throw in my own two cents for balancing. I'm a bit of a powergamer (*gasp*) so I have a fairly good idea of how powerful something will be in the game.

1) Only allow draining based on touch, as he absorbs the life energy through his fingers/teeth/whatever
2) Different people have different amounts of 'aura'. Epics usually have significantly more Aura than normies, making them better meals but also meaning they're harder to drain.
3) Draining continuously can drain a normy fully in 30 seconds flat, most epics require closer to 2 mins however. Drain speed can also be slowed down if Devilstrider wishes it to be. Epics with physical enhancement powers usually have even more Aura, requiring twice as long to fully drain.
4) During the night double draining speed, in absolute darkness quadruple it. That means that in absolute darkness he can drain an Epic in about 30 secs.
5) Using his other powers makes him more exhausted, and feeding can recharge him back to full. In addition to increasing the speed he heals at (although not to Wolverine levels).

These make sense to me, and should allow draining to be a highly viable combat option that becomes vastly more powerful at night. Just my input on the matter, feel free to ignore.