Well, the title says it. If Stoneskin remains capped at 350 HP, it soon will become totally worthless. While spells like Blink scale perfectly with the increasing strength of the enemies, since, no matter if the hit is of 10 or 400 damage, it will absorb two, stoneskin only absorbs a certain amount, and that amount is now less than what many NMs can deal in one hit.
Uncapping stoneskin would let it help soften the harder blows better, and absorb two or three lighter ones. I can't imagine how uncapping it would break the game, either, so I think it is totally reasonable to uncap it. Let the player's MND and Enhancing skill decide the size of the stoneskin, not some arbitrary cap designed in the 75 era.