Well, it would have a demographic impact, but there are Malthusian limits in Sub-Saharan Africa that are difficult to overcome. Lots of diseases and poor soil make life very hard.
But a lot of this can be dealt with by advanced development. That's why we have to be careful with the idea that Africa is overpopulated. It's overpopulated in the sense that there's more people than can be consistently supported by the agricultural output, but that's because they don't have advanced farming techniques or good transport systems to move grain about in bad years. I mean think of this without modern technology:
http://en.wikipedia.org/wiki/Great_American_Desert