I believe even the Mclaren F1 was tested here before its Euro release. Porsche Carrera GT, PRevious Boxster S, Mclaren Mercedes SLR, Noble M12 (body shells bits also manufactured in EC)etc....
Northern Cape is a particular hotspot for testing. I saw a few nice goodies in the VW Motorsport offices, but no cameras were allowed! I have also seen stuff in Clarens, Eastern Free State, where the 10km stretch to Golden Gate and the Lesotho Highlands Water Scheme revamped roads (Fouriesburg is a nice stretch too) provide excellent high speed testing twisties for performance cars under high altitude, high temperature testing grounds.
My fastest outing ever, driving a 996 Turbo, was on these roads, so if you are ever looking for a place to truly test your Quattro driving skills, its a good destination!
Perhaps we should have a club weekend there sometime, even the Overtaker might make the trip

Nice places to stay too!
Comments, committee?