Power Drain against the borg is basically useless. You can have a TR, ES and TS on a normal cube and its shields wont drop. The only thing you might achieve is to deactivate their weapons.
ES has the positive effect that you get your power up independently to the power drain.
borg can't lose all power. you can however make their shields a bit squishier
on my fed vo'quv i combine plasmonic leech with subsytem shield targeting, energy siphon, polarized disruptors and tyken's rift when i'm at or near 100 aux on a tac cube. the amount of damage to the shields increases and they start to drop faster for everyone. i do have 6 in flowcap skill, a 26 flow cap deflector and two 30 flowcap plasma infused consoles however.
against other npc types this pretty much shuts them down entirely.