I don't believe this topic has been answered by the devs yet.

And I fear it may be one of those topics that will be meant for players to discover.

From the different testing threads of attack power vs strength, attack magic potency vs intelligence, and such, the soft cap has not appear to have been reached. I don't believe anyone has reached the HP cap(s) for slotting HP materia all over the place either.

[Below I think I may be misunderstanding]
If I understand some of these damage posts correctly, there's a point where Strength and Intelligence don't yield the same efficiency against a certain level of mob.

i.e. To use 100% of your attack power against a Level 50 enemy, you need 250 Strength to overcome his 250 Vitality. If you have less his Vitality, say... only 220 Strength, you may end only doing 85% of your attack power against him. If you have 300 Strength, you'll still do 100% of your attack power.

I'm probably completely wrong on this, but that's what I thought I concluded from reading those threads.