I came in on that ET flight yesterday, and the immigration queue was completely empty - I was in the SLOW lounge in domestic departures 20 Minutes after doors opened, and that included getting my shoes shined. Admittedly I had no checked bags, but there are only a handful of intl flights arriving at that time, so I think that you will have plenty of time. For info, we left ADD about 25 minutes late and landed 5 minutes early.

PS even if you do get ET to through check the bags, you'll still need to immigrate and collect/recheck them in JNB as it's your first port of entry into South Africa.