I disagree on both points there, I don't think that an instanceof check is that costly compared to many of the other performance hits we're willing to accept with Clojure for saner behaviour. For example, persistent data structures are slower than mutable ones, but I'd never go back to using mutable ones in most cases. Why is one performance hit acceptable for better behaviour and the other isn't?

I also disagree about macros, too - I spoke at the conj last year about that, and that talk seems to have been at least part of the motivation for developing spec. I hope that things will improve as spec becomes more widely used, but right now the situation for macro error messages is pretty bad.