Anybody said Magic Coup?!
Love that kind of build, played it to exhaustion I reckon. The great thing about it is that you can use Stray Damage mod to splash hits on nearby opponents, making its LL effect much more interesting. I still recommend any given LL+% weapon on a secondary weapon slot for bosses with that build though. It's like you say, as its a percentual and non-boss mobs have a much lower HP value leeching a flat amount is more effective. Though at Platinum/Niob above level 100 the distance between the two begins to fade a bit.