Originally Posted by
Formalhaut
HP is maxed out at 9999, with Break HP Limit that rises to 99,999.
MP is maxed out at 999, with Break MP Limit that rises to 9999.
All remaining stats, which are Strength, Defense, Magic, Magic Defense, Agility, Evasion and Luck are maxed out at 255.
Adding more spheres that would take you above these limits would make them redundant. Having read walkthroughs and other guides out there, I see no reason to think you would benefit from "going beyond 255", simply because you can't!