Your macro does not consider the else case.
My initial one also did not:

(define-syntax CaseMetta
  (syntax-rules ()
    ((_ var ((a1 b1)))
     (case var ((a1) b1)))
    ((_ var ((a1 b1) ...))
     (case var ((a1) b1) ...))))

The complexity growth came from me trying to address the else case! :)