Iron ore->2 Iron Dust->2 Iron Bars->2 Iron Dust
As you can re-macerate the bars back to dust, this can cause an infinite loop of gaining material if it works with a Furnace.
Also seems a bit overpowered. Chance to compress two Uranium? Two Diamonds? It would also have to ignore anything that can't stack, as well.
According to the minecraft resource calculator, building a single efficiency module requires (among other things) 24 copper dust, 21 iron dust, and 9 tin dust. Because each module only gives a 1/32 chance of increasing your output, it would take 1,818 macerating operations just to break even. Although the possibility of acquiring an infinite supply of extremely common resources makes changing the recipe to require a mass fabricator and two recyclers seems tempting, as does changing the extra resource chance to 1/128. As for the unstackable thing, IIRC the only machine that produces an unstackable resource is the extractor (water cell -> coolant cell -> hydration cell).
Edit: Using buildcraft quarries, near-infinite amounts of copper, tin, and iron can be acquired anyway; it would take 512 coal dust, with the maximum four efficieny upgrades, to produce an extra diamond, and uranium is rare enough that (imho) there isn't enough raw ore to compress into an op number of multiple extra ingots anyway. In addition, most machines use 2 EU/t to process materials; setting the efficiency upgrade to consume an additional 20 multiplies the EU/t consumed by a factor of 11, which, if my understanding of IC2 energy is correct, increase the energy consumed by a single operation from 625 EU total to 3,438 total.