In a quick look-around for various months on ITA, I can only see a couple of origin/destination points and carriers (e.g. DXB-JNB, CAI-JNB on KQ or SA, nowhere to CPT) where the coach fare is less than the (NRT) DONE4/DONE5 differential, and nothing comes even close in business class. Not saying cheaper fares aren't possible, just not that I could see in a cursory scan. If the OP's intent is to requalify for EXP, then the points harvest might be worth the slight differential.