Changing at JNB to a domestic flight is no picnic. Been there, done that
Expect to collect luggage, immigration, recheck luggage, long security lines and a at best mediocre SAA domestic lounge (though I believe this has recently been renovated), assuming you are connecting to SAA.
Unless you have a fully flexible full fare ticket I doubt you can get IST-JNB-CPT easily changed to IST-CPT. If you are consider this I would use the ticket counter and not the lounge.