That is hard to imagine for me. :/
Basically it's streaming the input like this:
Wii U Gamepad > Wii U Console > Router > Internet > PC
I'd guess that at least when playing games that require precise timing you'd notice something.
Sort of. It's all over your LAN, so the time it takes to communicate across your LAN on a local server is really minimal. It's all dependent on how busy your local network is, but I can honestly say I've not noticed any lag in some fairly precise timing scenarios. Of course, each setup and scenario will differ, so I welcome your input after testing.
P.S. Sorry to all about the delay for the mouse controls. They're completely done and tested, I just haven't packaged it up and uploaded yet. I'll hopefully do that in the morning for your enjoyment! Also, don't worry: I hear all of the cries for a more thorough tutorial/simplified installation process. I'll whip something up to help the masses enjoy the product more rapidly.