[unfoundnemo]

Aside from the points mentioned by Blooze (exhaust and interior), the M8C also includes the extended shadowline components (grille, side vents, mirror caps, spoiler, and badges), the obvious wheel difference, M colored seat belts, along with some different engine mounts and bracing that will stiffen the vehicle up. As mentioned, full leather + Exhaust what makes it worth it, and that's over 10k of that 13k difference.

Granted if you're trying to find a coupe or convertible and you're seeing the limited inventory, the base M8 will make you smile just the same. You could add the shadowline components (or perhaps carbon instead), get an exhaust, wheels, etc.. Basically beyond the full leather, the extra HP through a tune, engine mounts, and the bracing are the only kinds of things you'd still be missing without modding more heavily, and loosing those items would only be marginally noticeable at the limits.