I use the alcohol/water mixture as a clay lubricant. It evaporates quickly, so the clay remains dry and doesn't become mushy. You do need to spray more than usual, but it's cheap and wipes off easily. It doesn't add any extra slickness... QD's often make a non-clayed section slick as well. IPA/water really helps deep clean the paint.
I don't have a specific dilution ratio... just a little more water than alcohol. It works fine for me!