You don't NEED a bigger board for those powder days but you will definately WANT a little bigger board. Most of the locals up here tend to ride twins or directional twins in a little larger size for the sweet, sweet pow. It's all preference but with rocker designs there isn't much need for a lot of taper in our Sierra Cement. From what I understand after dealing with Kiwi's for years the prices in America are much better than those in New Zealand. The shops in town have plenty of old boards in stock so grabbing a larger board once you get here makes the most sense. Plus it gives you the chance to see what Mammoth conditions are all about before deciding on the perfect shape for your riding style.
I have a pretty amazing hook-up so my quiver is really perfectly suited for my riding style. I'm 6' 195 pounds and I ride- either a 57 TRice or 59 Attack Banana for groomer/park/freeride days depending on my mood. For powder days I almost always ride a 64 Horsepower TRice, unless its a "storm of the century" dump in which case I ride the Grocer (180).
Hope this helps out!