What penalty is exactly incurred if you wear gear over your actual rank?

For example, a R10 CON wears a R26 Canvas Robe favoring CON at stats MP:+37 MND:+2 Craft:+11 M. Craft:+7.

Does the player receive a percentage of these stats or none at all? Does the stat/rank difference take effect at 1+Optimal Rank or a higher threshold?