I just tested it on Streetmaps 2009 (If you have a new Zumo I suppose that's what you've got as well) and it knows about all the roads.
Here's a Mapsource file of that trip.
The 'route' is what CJ set up before the trip.
The Track logs are from my GPS after the day - we deviated in the sense that we came back over BainsKloof and not the same road (through Rawsonville) as we went.
My GPS didn't like the tunnel and then I switched it off to only start it up again in Ceres - so the log only starts there as well, but luckily that isn't the interesting bit yet.
;D ;D