Regularity of solutions to Kolmogorov equation with Gilbarg-Serrin matrix

In R-d, d >= 3, consider the divergence and the non-divergence form operators -Delta - del . (a - I) . del + b . del, -Delta - (a - I) . del(2) + b . del, where the second-order perturbations are given by the matrix a - I = c vertical bar x vertical bar(-2)x circle times x, c > -1. The vector field b : R-d -> R-d is form-bounded with form-bound delta > 0. (This includes vector fields with entries in L-d, as well as vector fields having critical-order singularities.) We characterize quantitative dependence on c and delta of the L-q -> W-1,W-qd/(d-2) regularity of solutions of the corresponding elliptic and parabolic equations in L-q, q >= 2 boolean OR (d - 2).