What's the point of getting 100 smithing, taking the dragonscale/plate perk but not being able to get extra XP for smithing because you have already reached 100?
Personally, I think its better if you've taken the light armor branch for smithing and get glass smithing perk, obtain potions to boost smithing temporarily, smith as much as possible with it and enjoy the benefits. I've done this, and my AR ends up higher than having dragonscale armor and smithing that to legendary status!