I did notice some improvements, but nothing too big.
Wii mode on Wii U does not fill the entire screen.
Wii does.
Someone already explained the whole ratio deal. So I am assuming the Wii has always been "stretched" even on Widescreen mode.
Colors are more vibrant. I guess that's about it. MPTrilogy still looks glorious though. Replaying on Wii U right now. No HUD, no visor. *drools*