I did more research yesterday, and I found that the model of Kay on the new box is actually different than the one used in the prototype DS box which had the same pose:
The poly count on the new box is far higher and the textures are also far better.
However, the models of the other characters on the background are the same as on this DS box, and not much different than the in-game models used as artwork in the original PS2 version.
Point is, I'm thinking Kay's model on the Wii U box is indicative on what kind of an upgrade we can expect. They at least seemed to have beefed up his model so hopefully it's in-game as it doesn't look CG.