To be honest, it's not predictable, unless the images have a consistency of color, contrast, density, and subject that make it so. That's why I say that large-scale printing has to know what it's doing regarding all sorts of technique - interpolation, masking, edge enhancement (sometimes referred to as sharpening, but not quite the same as sharpening for normal size prints), and so on. I've seen marvelous things done with small negative films and enlargers with different light sources, but digital is not the same, since pixels have a regular size and shape unlike film grain, and are harder to disguise in the final print. Be prepared to spend a lot for good results. In case I forgot to mention, the 8 -vs- 13 mp difference won't be nearly as much a factor as the other factors I mentioned.