I was at the talk in question. Some more context:

Bignums "contaminate" surrounding operations. Adding a long and a bignum yields a bignum. Seeding an equation with a single bignum (42N is a bignum representation of 42) will prevent overflow.

No one has come forward with a single real-world scenario where they're using bignums in Clojure. Choosing a default that is only theoretically useful rather than a 10x performance improvement seems a bit silly.

In any situation where the compiler cannot be sure you're using primitives everywhere, it will emit non-overflowing bytecode.

By my measure, there's nothing premature about this optimization. In Rich Hickey's words, Clojure is a replacement for Java, not Ruby. Giving up Java-like performance makes the language quantitatively less useful.