I am not sure which part of JHB you live in but the western side towards Magalies and the Dam have some pretty good roads. Other than the obvious R512 (Lanseria Road), which is pretty dangerous, the satellite road (R400) is probably the best one for high speed, with an awesome straight past the satellite dishes which will allow 340+kmh on a ZX-12R if you hook 6th gear before the kink...
Then there is the R104, closer to Rustenburg past the buffelspoort dam, which is pretty good. If you want to go further out then the R30, from Derby to Olifants Nek, is quite good as well (especially early morning).
Not sure what type of riding you do, but there are some really good roads in that part of JHB, with pretty much anything around Magaliesberg, Rustenburg, and Harebeespoort providing good fun.
As always, make sure your bike is well prepared, your phone has some charge, and that you are wearing the proper gear before twisting that grip to the stop.