Comparing it to vanilla JS is the pointless part to me, that's like comparing a appending a list, and diff algorithms together, with complicated way to say it.
Of course it is a lot of work to diff two DOM trees, but the general impression on React was that the DOM is soo unbelievably slow that it will still be much faster to do the diffing (which i never understood, technically, but what the heck, FB engineers are wizards!). At least, that was my impression i got from the React announcements. And i don't even code JS nowadays, so i couldn't care less if you prefer React or Ext or write everything as Silverlight plugin :P
Nevertheless, if you ignore the code complexity, i am fairly certain you can write JS code to update something without needing to traverse the whole DOM each time. I don't program JS, but wouldn't you have some variable holding the reference to the place you want to update? Like you have a DOM element that will be updated every second, you would surely not search the whole DOM every second, but get it once and udpate it multiple times? No?
If you have a table and want to edit information this is trivial to do in a React like way without React. Edit button stores reference to the row, edit values, update data store, render out new row element, remove element, insert new element. Yes, React will generally be less code and the argument becomes whether this is more complex, which it is, but I'd say neither the complexity or performance truly suffer. I bet the performance will be faster as you removed diffing entirely and React will have to perform your logic anyway.
This would cast doubts on some of the theoretic claims on the React docs, at least for mobile.