I've only played with react wrapped in reagent (cljs), but seeing the underlying code here, I feel comfortable saying you can benefit from this pattern relatively early. Just having the virtual dom/view-as-projection-of-state concept saves you a lot of mental overhead about state management. If you're going to build an SPA and you're not already totally sold on yourFavoriteJSFramework, give react a try.

I've actually found working with react/reagent to be substantially simpler to reason about, and lower complexity pays dividends far before any performance or scalability gains do (though I hear react is quite good in that respect as well).