Do you mean "most successful in America?" Yes, they are probably that, though Tree63 could give them a run for their money. The Springbok Nude Girls were forced with the decision "go to America or break up" and chose to break up, but reunited when solo efforts weren't paying the bills.
Another South African band in the US is Henry Ate (Miami), and if you do run in the Christian music circles, Jeremy Camp's wife is South African; she was lead singer of The Benjamin Gate until she met Camp while touring with them and I guess the band didn't survive her having two babies and being a wife. She tours with Camp singing back-up, I think, and may have released a CD on her own.