The outbound would not be a problem as the flight leaves in the evening and you can take a nap after "dinner". The problem usually is coming back. The headwinds are quite strong and you feel like the aircraft is stationary. Either way prepare yourself by:
1. Skipping sleep for 1 or 2 nights before the flight so that you are physically tired and will nap right away upon boarding the flight. Sleeping the entire duration is the only way to get over this flight.
2. Bring adequate amount of water for the ATL-JNB portion as Delta will usually run out of liquids on this flight. The JNB-ATL, drink plenty of water before you board the flight as they will confiscate all your water before boarding.
3. JNB immigration is a breeze when Delta lands. At the stated time of arrival you dont have the European bank of flights arriving (usually mornings) which makes it an extremely long wait. Same goes for departure. The European flights leave after Delta so exit immigration is also quick.
4. JNB customs is also quite friendly. Your bags normally show up by the time you finish immigration.

For folks who wonder how people do this in Economy, I would take an exit row or bulk head seat anytime over a Delta "coffin" seat. The "coffin" class seat does not fit me nor allows me to roll onto my side. It does feel like sleeping in a coffin with god awful service from the FAs. If it is SQ business class then it is a no brainer. With Delta, you are better off with the exit row or bulk head seat.

P.S. Yes, I have flown DL 200/201 in business so have I tried their other aircraft like the 767 in business. It sucks!