That's an interesting one. With the increased agility of the mullet rear end I did find that I was being more aggressive with the rear end and squaring off corners more for example. It would follow from this that you would need a tough wheel and a heavier casing. As I mentioned in the article I ending up running a full DH casing in the rear. That tire combined with the super strong Reserve carbon rims meant the back end was very 'tough'.
My opinion is that traction is definitely better with the 29er (all else being equal). My son races World Cups and one of the reasons those guys love the 29er DH bike is the improved traction.