Does armor have a cap on how tough it can be still? I've downgraded to Stalhrim and wanna know if that changes anything. And if there is a cap, what is it?

The armor cap used to be 567 AR. Whether or not it is changed I cannot say for sure, but I should think it is the same still.

the armor cap of 567 armor rating, or 80% damage resistance, still exists.

Thanks guys.