Yes, I too can vouch for Wayne or Johan at Auto Atlantic Motorcycles (the official Cape Town BMW dealership. Their telephone number is +27 21 4095831. They are very good and extremely reasonably priced. I have heard fantastic things about Trefco from other travelers, but also experienced some bad service there first-hand (as did some other people I know).
If you go to www.bmwmotorrad.co.za,
you can follow a link that lists all the BMW agents in South Africa. Generally speaking, the motorcycles are very well supported by the dealership network in this country.
Best of luck, enjoy the trip.
Cape Town, South Africa