The Cleansing of Saidin was considered such a threat by Moridin that he rescinded his orders not to kill Rand.

EDIT: Forgot to answer the second half of the question. The Gathering Storm did not change that much, the only real damage done to the Shadow in that book was eliminating some of its servants - Rand killing Semirhage and Garendal, and Egwene scourging the Black Ajah. Given that I expect Moridin accounted for the two Forsaken's death, and that Mesaana has demonstrated an absolute lack of concern for the Black Ajah (though this is probably her own flaw, Shaidar Haran was apparently more interested).

I'd say the purge of the Black Ajah removed a considerable resource for Shai'tan's use in the Last Battle, but it's still inconsequential compared to the Cleansing of Saidin.