I own the VSM-MMs, and I noticed breakin inprovements out to around 200 hours with perhaps a majority of the breaking in around 100. In short, I would say that YES the improvement is dramatic. Bass deepened, highs extended, soundstage deepened, and imaging improved. When I heard the merlins right out of the box, my jaw LITERALLY dropped. They only got better, much, much better beyond that.
Let me know if I can answer any other questions,
Jason