(define-syntax +

   (lambda (stx)

      (syntax-case stx ()

         ((_ x y) #'(fast-binary-plus x y)

         ((_ arg ...) #'(slow-general-plus arg ...))

         ((id (identifier? #'id) #'slow-general-plus))))

..otherwise, the (_ #'slow-general-plus) clause will match the entire form (+ a b c d ...), not just the identifier +, and the result of expression will be the slow-general-plus procedure object.

This should also resolve John's concern.