Just bought Luigi's Mansion 2 today, thats a pretty good opening week for the game!
The original on GC only sold 457k - http://www.vgchartz.com/game/1207/luigis-mansion/Japan/ - On an opening of 160k... So this is brilliant.
It sold well on its own. Just expectations were inflated due to high shipment numbers. 3DS and GC aren't much of a comparison though.