The effect of increased concentration is similar to temperature increase, although the effect of temperature is more important than that of concentration. The increase in concentration limits the maximum amount of particles you can get due to the higher dissolving power of the concentrated solutions. Maybe this MSc thesis can help you: http://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=4508&context=etd_theses
However, Shingubara has shown the greater electrolyte concentration, the better pores arrangement, what means better spatial distribution of the electric field lines. Moreover temperature increase also enhances current density (see my paper from Surf Coat Technolg, availabla on researchgate).