Ember's approach to watching for changes is pretty different. Also, the Ember view layer refactor codenamed HTMLbars has yielded some extremely promising (though admittedly early/premature) performance improvements that often outperform the React equivalent. So we're open to the idea and we have much we can learn from each other but we're not going to merge in React if we can do a better job :)
For me, and I think most people who get excited about it, performance isn't what attracts me to React. It is the semantics (is that even the right word?). The nice way it lets me think about things.
It looks to me like htmlbars is handlebars emitting DOM rather than string. In that case, I think it will be missing the benefits of React. Again, though, I obviously don't understand Ember well.