Hi Vanja,
although I've only used the Heathrow - Jo'burg route once (about 3 years ago) I found it to be a reasonably good experience. My Father uses SAA 2 - 3 times a year on the same route, and prefers them to the other providers based on service (he found virgin were really bad) and price.
As for turbulence is concerned, I don't think he has ever experienced any turbulence worth mentioning. I hope this little bit of experience helps.