I would advise against attempting this. You might miss this connecting flight altogether, let alone your bags not making it. I flew from JFK last July on SAA and had a 3 hour connection to Luanda, Angola. I barely made it and my checked bag did not. It took 7 days for them to get me my bag. When the plane lands at JNB, it parks out on the tarmac and you have to wait for a bus to take you to a terminal. Even though you are "in transit" to another international destination, you still clear through passport control, an abbreviated version of immigration. It can be a slow process.