Much depends on what Smuts can do with it, and whether it results in butterflied apartheid or what.
Combining all of that into the Republic of South Africa more than doubles its land area and adds to many of its problems its going to face with black Africans. South Africa has nine black tribes, and you've just added at the very least half a dozen more. What does this do to apartheid (and the struggle against it) is a good question. What this also does to the world's mineral markets, particularly since SA now has a practical stranglehold on the world's diamond market, is another good question.
I think to get a good answer to your question, M79, you need to stipulate a few conditions, namely what the world's political climate is at the time and whether Smuts can do this without serious political problems to himself and others.