You can't raise your stats to an overflow with the grid or even the super bosses would deal ridiculous damage and they don't. And that would be easy because getting Def and MDef Spheres is easy and fast to fairly easy and fast later. For the additional support you have Protect, Shell and Def and MDef + x%. I always have overflowing stats, in terms of the count on my grid being over 255 but they don't actually have an effect.