More research says AGOA had a strong positive impact:

While imports in the key product categories covered by AGOA (apparel, and a large number of agricultural and manufactured products) increased 94% in the post-AGOA period, we are able to discern that the portion of this increase actually caused by AGOA was much smaller – closer to 34%…

We moreover find that the increase in U.S. imports was not the result of a decrease in European imports. In fact, for manufactured products the Act even appears to have led to an increase in imports into Europe for the AGOA products. While this could be related to a number of factors, it certainly is consistent with the presence of fixed costs to exporting…

Eliminating trade restrictions on apparel products that initially faced a 20-25% tariff caused a four times larger import response than eliminating tariffs in the 10-15% range. It suggests that prior to the Act these tariffs had been quite effective in keeping out imports…

Overall, AGOA resulted in an 8.0% increase in non-oil exports from AGOA countries to the U.S…

the other limitations frequently cited in the African context – poor infrastructure, distorted product and credit markets, high risk, inadequate social capital, and poor public services – did not turn out to be binding constraints to expanding exports under AGOA.