[ANTSANTS]

Maybe it was a bad example, but in either case, I would expect a "sufficiently smart compiler" to transform

  (defun fun (a &key (b 0) (c 0)) ...)
  (fun 5 :b 3)

into something like

  (defun fun (a b c) ...)
  (fun 5 3 0)

Whether it does so as a regular macro or as a special form of the compiler doesn't matter, there's no need for there to be any difference in efficiency between representing keyword args with tables or plists.

As I later mentioned in another comment, it was a mistake to talk about hash tables when I was really more interested in the abstract map datatype.