Not horribly OP, but Mimichite works differently on pulverised ores than it does on ingots. As long as your supply holds out, you can get 4x ores with a pulveriser... Very useful in early game when you're short on a particular ore type. I was using it last night to compensate for a gold shortage while building an initial TE setup in a new world.
To duplicate a gold ingot requires 4 Mimichite, but to duplicate gold dust or pulverised gold only requires 1. From what I can tell, it applies for crushed ores, purified crushed ores, ore dusts, dirt ore gravel, clean ore gravel, reduced ore chunks, and pulverised ores. For aluminum, iron, copper, and tin it doesn't matter... 1 Mimichite doubles either the ingot or the dust. For silver and lead, the ingot requires 2 Mimichite, but the dust only takes 1.
With a Factorization setup, you can lacerate, wash, slag furnace, crystallize... and then double the crystalline ore into 2 ingots at a cost of 1 Mimichite. Of course, by the time you have all that processing up and running, you should have no shortage of anything anyway.
In FTB, the recipe for creating mimichite has been drastically reduced in output... A decision with which I agree. Even so, with the exception of the ghast tear the recipe components are easily farmable (lapis, glowstone dust, gunpowder, magma cream, ghast tear = 4 mimichite). I think you can even farm the ghast tear with magical crops.