How is Steel Plate armor, with 100 smithing, and 5/5 juggernaut and the other buffs in the heavy tree for the full set. What's the armor rating? Also. What's the damage for a steel greatsword with 5/5 barbarian perk with 100 smithing at legendary status, compared to a daedric greatsword with the same perks/ skill level.

You could easily get Steel Plate to reach the armour cap with 100 Smithing. A Steel Greatsword will do a healthy amount of damage once upgraded at 100 Smithing.

With 100 smithing and advanved armor tagged & 100 heavy armor with 5/5 juggernaut + well fitted and matching set, the armor rating will be 603. A teensy bit above the cap.

When improving armor and weapons, the bonus they get is the same no matter the material (half bonus if you don't have a perk in the relevant material).

A daedric greatsword's basedamage is 24 while a steel one does 17. The delta will never change (again, assuming you have the relevant perks). If you smith your daedric sword to do a million damage, the steel one would be just behind.