If it was another date, away from peak season, I would consider 'self-insuring', booking that Safair flight and assuming if the BA flight is too late there are so many flights and carriers to CT I would suck up the extra cost of re-booking. However on the Friday before Christmas, in the first week of SA school holidays and everyone trying to visit family, you can't be sure of any space on later flights nevermind the walk up cost. That Fri/Sat/Sun are likely highest demand on that route all year. Is the consequence of not making it on the Friday severe? I would dislike it, but if you can't easily amend to a Cape Town flight then for me I would probably book a later transfer flight to be 'extra sure'.

The BA flight is late very-approximately once a week at the moment and I just don't see BA fixing the A380 issues this year. In Feb we were about 1.5 hrs late arriving and then lost 20 mins to load shedding when collecting luggage. On the other hand you may be lucky and everything goes easy and you are stuck wasting time. Such are the perils of separate tickets.