Effect of spatial variations of superconducting gap on suppression of   the transition temperature by impurities
  We calculate correction to the critical temperature of a dirty superconductor, which results from the local variations of the gap function near impurity sites. This correction is of the order of T_c/E_F and becomes important for short-coherence length superconductors. It generally reduces a pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic impurities can increase the transition temperature. 
M. E. Zhitomirsky and M. B. Walker