I have taken SAA from IAD-JNB and it was very good. You do not really want to end on JNB, but perhaps Cape Town. Or go north for a great safari.
Remember that US will probably be leaving Star Alliance so keep an eye on those dates and get your tickets before the turn over. If you wait for oneworld to Africa you will pay huge fuel surcharges for British Airways. You will lobe South Africa.
I have take Turkish Business product and I think it is better then LH.