We characterize the transport of z-z electrolytes embedded in varying cross section microchannels with inhomogenous slip on their walls. We show that, within linear reponse and Debye-Hückel regime the transport coefficients, and so the fluxes, can be substantially improved by the presence of a hydrophobic surface coating located at the narrowest section of the channel. Our model indicates that the enhancement is larger for conductive than dielectric walls. We show that this is produced by a synergy between the entropic effects due to the geometry and the presence of the slip boundary layer. Our results suggest that performing a fine-tuning of the slippage on microchannel walls allows for a significantly enhanced performance of blue-energy technologies, microfluidics-based energy harvesting or water desalination devices.