Recent work has shown that type-I superconductors may undergo genuine wetting or interface delocalization transitions. These transitions arise when superconductivity is enhanced at the surface or at an internal defect plane. Experimental verification of the effects associated with these wetting transitions is possible and has to some extent already been achieved in twinning-plane superconductors, such as Sn, Here we derive an expression for the wall/superconductor interfacial tension Gamma(W/SC), Valid when the Ginzburg-Landau parameter kappa is small. This expression is essential in locating the low-kappa interface delocalization phase boundary accurately, and thus allows us to complement the previous numerical results for the wetting phase diagram. We find that the phase boundary is singular in the limit kappa --> 0, in the parabolic manner kappa proportional to (tau - tau*)(2), where tau = xi/b is the ratio of the bulk coherence length to the surface extrapolation length, We furthermore calculate the superconducting order parameter and vector potential profiles and verify numerically that the low-kappa expansion provides an accurate approximation across a broad range of kappa values.