I don't believe so. From what I've heard, 255 is a definite hard cap on all stats that can get that far. You're right about Agility being pretty useless beyond a certain point. Once you're in the 150+ region, you're pretty much as fast as you need to be and any more would be a waste of nodes.

But I wouldn't bother pushing Strength and Magic beyond 255, even if the game allowed you to. If you really want an uber team and you're doing this on the HD Remaster + Standard Sphere Grid, you've got the nodes to go for as high a Luck as possible (255 is unreasonable - it's a lot of Fortune Spheres) and that way you're getting yourself an awesome Critical Hit rate.