Clojure and ClojureScript data structures use 32 way branching nodes. This means even with >2 billion elements in say a persistent vector you'll never have more than 7 hops on any particularly path. This means in the worst case you will need to copy and update 7 children all which are small arrays.

I did a bunch of GC testing with Om, the GC profile wasn't dramatically different than naive Backbone.js on TodoMVC which is also working GC by blowing away the DOM on every update.