What kind of performance hit does ClojureScript come with? I imagine there's a lot more of an impedance mismatch between Clojure and JavaScript, especially since you presumably only use immutable objects and arrays.
ClojureScript adopts some of JS semantics like the numbers. Like with Java you can use nativ arrays if you really need to. ClojureScript has an implementation of Clojures immutable Data Structurs this are slower then nativ objects but should not be the bottleneck in most applications.
The great thing about ClojureScript and performace is that ClojureScript get piped threw the google closure compiler with does dead-code elimination and some other optimizations.
I have not written any big enought app in ClojureScript that I could really tell but if you look at the compiler output you'll be surpised how little there is.
