Does raising smithing over 100 do anything?

Yes, smithing over 100 has an affect on upgrading. In this answer I raised smithing to 130 via two items. That allowed me to increase the armor rating from 528 (@100 smithing) to 610 (@130 smithing).

Raising smithing above an effective value of 100 using items or potions does increase how much you can upgrade your items. Although there is no higher category than Legendary, you can still increase the numerical bonus.

You can test this by upgrading an item to Legendary without any gear or potions, then equipping items of +Smithing and drinking a potion, and upgrading the item on the workbench or grindstone (quickly, as the potion time limit does run down while you're smithing). You'll be able to upgrade a Legendary item even further.