I'm currently using Steel Plate Armor (Legendary) with 191 AR and 38 weight. Crafting Daedric Armor (Legendary) would give me an additional 67 points of AR, but it has a weight of 50. I have about 500 AR which is already near the cap. Is the extra armor worth it?
There are two things to consider. The first is that Armor Rating in Skyrim has Increasing Returns - that is, the more you have, the better it protects you. So to answer your second question first, the 67 points you're shy of the cap will get you more mitigation than any other 67 points.
It sounds like you should be able to reach the cap with a little bit of Fortify Smithing Gear / Potions.
According to the UESP and my rough calculations, you only need:
67/4 = 16.75/.2 = 84% fortify smithing (This is assuming your smithing is 91 w/ the perk. You need less if you have smithing at 100) which is easily attainable with items / potions.
So with a little investment, you can hit the armor cap with your steel plate.
To answer your first question, once you're at the armor cap, Daedric really isn't better than Steel Plate (unless you like the look) especially because it's heavier.