Lately I've been wondering if there's any benefit in creating a set of armor that gives you +skill in any skillpoint over 100.

To give an example, let's say you have 100 skill in Light Armor and craft a set of Light Armor that gives +25 to the Light Armor Skill (4 pieces giving you +100 for a total Light Armor Skill of 200).

My question is: is that set of Light Armor going to give you more damage reduction than a regular set, or would smithing a regular set up to legendary be better? And is there really any benefit to going over 100 skillpoints in any one skill?

Smithing up to legendary would be better. If your smithing goes over 100, for example, you'd be able to get a few extra armor (micro-)points added via tempering. One-handed would let you do a tiny bit extra damage.