Quick A: The default deleter does not store anything.
Recently on SO:
std::unique_ptr<T> is quite likely to be zero-overhead (with any sane standard-library implementation). std::unique_ptr<T, D>, for an arbitrary D, is not in general zero-overhead.
The reason is simple: Empty-Base Optimisation can be used to eliminate storage of the deleter in case it's an empty (and thus stateless) type (such as std::default_delete instantiations).