Nintendo has learned to SCALE after the christmas debacle. They can setup within a MINUTE new servers. I guess they might even use Amazon EC2 for this.
They are prepared. The only weak spot I could think of is the actual selling process within the eShop.
The bandwidth hungry task of providing eShop downloads - Nintendo seems to be pretty shure they can handle that. Otherwise they won't offer a demo. So they can handle also the actual payed content delivery.
On the gaming side MK8 is a mostly "harmless" besides the voice chats. But these can be calculated and as all streams of a private game could be handled on one server - this scales also very well.
No question - it will be the toughest release for Nintendo in their history. But I'm confident that Nintendo will be able to handle that and give us the best MK 8 buying and gaming experience possible.