Photo realistic images in my opinion really come down to the separations. They can actaully be easier to print than spot colors as the registration errors don't show up as much because of the smaller dots. You do need to use a higher mesh count and LPI and also be able to print halftones with your film positives, but I have seen some amazing prints using just 4 to 6 colors.
Check out a few samples of these kinds of prints that were done with the color separation software,
FastFilms.