It can only help to ease the hardware bottleneck of the wii.
For me, it seemed that lag was generally improved, though it was minor. People seemed to move a lot more twichily, as if the game was able to correct peoples location at a higher rate. Instead of smoothly wandering over larger distances.
I totally agree with Chain that any improvement in lag is pretty minimal.
I have also seen the twitchiness that he described. I'm not sure that it's a good thing for me. It's made it much more difficult for me to track targets using the PR and Scar while scoped. I was always lagging behind the target. In fact, after using the default turning speed and cursor sensitivity settings for more than a year and a half, I had to increase both by about 15% to try and compensate for the twitchiness.
I also noticed this twitchiness in some of the players here when they got their WiiU's even while I was still playing on the Wii.
I'm not sure how the Wii U runs the Wii games, but it may or may not be able to utilize the additional processing power of the U.
I will say that my Wii U has fewer problems connecting to my WiFi network at home. I was running my Wii with a LAN adapter.