The average index of export diversification in SSA is 2.2 (the index measures the extent to which exports are diversified; 0 low to 100 high).
2.2 out of 100 sounds really low, but I’m having difficulty finding data indicating the average index value for other regions.
Trade Diversion 
See UNCTAD’s Handbook of Statistics (the link to data “International merchandise trade indicators “).
It’s an ad hoc measure in any case. Broda Greenfield & Weinstein have a more rigorous approach: http://faculty.chicagobooth.edu/christian.broda/website/research/unrestricted/BrodaGroundnuts.pdf. They only cover a few African countries, which are clearly less diversified than the rest, but not to the extent that this 2.2 / 100 would seem to suggest.