WiiU wireless is 1ms as stated by Ubisoft (Rayman) devs. It's also running on 802.11N. I've sat and played/watched Mario on the bigscreen while using peripheral vision to keep my eye on the gamepad screen. IF you know what you're looking for, you see the barely noticeable lag. If not, the tv and gamepad screens are pretty much in sync with each other.
This was actually one of the first things I actively looked for after I booted it up, updated and played the new shiny Mario game. I really wanted to see how well the gamepad held up and it did pretty damn well.