I don't know exactly how it works, but I don't think it works quite the same way.
Case in point: taking 13 shield skill makes your shield unbreakable as resistance goes above 100%. This works with difficulty 0 shields as it does difficulty 6 shields, so there doesn't seem to be a cap of benefit above the shield's requirement.
If you're asking if having 5 shield skill might make a difficulty 3 shield more beneficial than a difficulty 5 shield due to the 'bonus' from the extra 2 shield skill I don't have a definitive answer for you.