From what I've learned on Gamefaqs, HD Namine and co work as follows:
Their Keyblade Multiplier and Special Attack multiplier are used to determine damage.
The Element, Special Ability, and Target Radius of the copied Medal are used to determine the attack's form. Not sure about Skill or Trait.
So basically, If you have, say, HD Namine on slot 4 of DR and Marluxia B on slot 5, Slot 4's multiplier and Namine's Guilt% would affect the damage, but it would be a single hit Speed type AoE attack. Switching Marluxia B for Xigbar B would make it a Single Target 6 hit Speed Type attack with the GDD-2 effect that ignore attributes.
So it literally copies the element, ability, and target type of the medal being copied, but relies on Namine's own Special Attack multiplier for damage output. So unless max Guilted, Namine, Oogie, and the Key Arts with the ability wouldn't do exactly the same damage, but assuming Namine's own Traits factor in as well, if she were to get EA40, you could theorectically use the copied medal 3-4 times (4 if the copied medal has EA40 as well, since it would have 2 uses). Not as massive for damage medal as it is for buffers/debuffers, since you'd be able to stack the effect to ridiculous levels with the right setup.
So they're not really gamechanging per se since you need to Guilt them for best damage output, but used right can be a potent force in battles and in Proud Mode, as Namine being Single Target means you can use her special in Single Target restricted missions, but copy an AoE medal and still hit everything without worry. Could be very handy.
There's part of me hoping Oogie becomes another VIP Medal as a result, since if he goes in a HSC I don't see Guilting him to be as easy...or rolling the right trait.