Where do some of ya'll get the idea that EWR/JFK-HKG and JFK-JNB are longer distancewise than ATL-JNB? I checked on distances and flying times between the city pairs (on CRS) and according to everything I've found, SAA's ATL-JNB 744 service will still be the longest flight out there by several hundred miles. If you're talking timewise, well yes, that's a different story given the prevailing winds. But really, that's a somewhat more dubious distinction, eh? I flew ATL-CPT a year ago and that was PLENTY long for me at almost 15 hours.
Anyway, for the record, according to Delta Air Lines CRS, ATL-JNB is 8,439 statute miles. JFK-JNB is 7,973 SMs. I don't remember the exact mileage EWR/JFK-HKG but it's right around 8,000 SMs (I'll write it down next time I'm at work). For comparison, UA's ORD-HKG is 7,788 SMs. LAX-SYD is similar, not quite 8,000 SMs.