The $43bn figure (the amount the World Bank (WB) lent in 2011) can be found on the WB website here, the factor of 17000 comes (I think) from dividing $43 bn by the expected annual donations from the pledges ($43 bn / ($112 mn in pledges / 45 years of work) ~ 17000).

However, obviously, as you state, doubling the effectiveness of WB activities will not have the same impact as bringing CEA up to the size of the WB, unless one (unrealistically) assumes that the GWWC recommended charities are only twice as effective as the average WB intervention (though ideally one should take into account the diminishing marginal returns of GWWC and 80k).