It's actually interesting about the underscan stuff. Call of Duty: Black Ops II on the Wii U has underscan that shows up on the GamePad, when using the composite (or, is it the component?) cables on a SD TV.
I had played Nintendo Land, Sonic Racing on the Wii U, on a regular SD TV, but the games displayed on the FULL GamePad screen...Black Ops II was the only one I experienced to use only a portion of the GamePad screen with the regular cables.
I just thought it was the way they programmed it, so it would run smoothly on both. I had no clue the FULL GamePad screen would be used when we upgraded TVs in January...that really did make a difference in the online multiplayer (which I had been playing already on just the GamePad), as the FULL GamePad screen was in use.
I'm not sure if this video might help, but I posted up
on my piece in the comments section when somebody mentioned the overscan issue:
http://www.youtube.com/watch?v=efXNTPiw6sA