The reason why Mori scales better is because it implements arrays via a tree of 32-slot arrays. So unfortunately, you cannot use normal JavaScript data structures and have to re-implement all the array methods that know how to deal with this data structure.
React.addons.update uses normal JavaScript arrays. So it won't scale as well, but at least you get immutability.
If your react state contains large lists, I think React's render() is going to be the bottleneck, not javascript arrays. That's why react state needs to be structured in a tree, so you can implement shouldComponentUpdate and get O(log n) renders. I wrote more in reply to jlongster.
React.addons.update uses normal JavaScript arrays. So it won't scale as well, but at least you get immutability.