Mirrors arent a long encounter. However, they still are harsh on healer mana due to keeping of two tanks with 5 healers. Currently, they are basically perfectly balanced, since they are doable, but not without going OOM on our healers. So no, your reasoning isnt good enough, because although they may drop fast, they always have dropped fast and the challenge lies in holding agro/keeping tanks up. The reason you are able to heal the way you do is obvious, Vargg and Volkov are overgeared for Mirrors.