@steve_fox was one period that I was studying a lot to increase the WIfi between the Pad and Wii U.....
After many studies I found two solutions for it, and was a modding that I would like to make here a guide, but if you arrive first you are welcome.
The solution for the 5Ghz and 802.11n it's to looking for the Antenna pre-amplifying (plugged to the electricity) and increase the dB / the range of the signal..... Something like the picture below. Or to looking for the antennas of Drones or bigger as you found, but the problem of this Drone solution or just bigger Antenna (as per my understanding), you will increase a bit the distance but you as most increase the stabilization of the signal without go more far away because the problem it's the frequency at 802.11n, that it's not change the range as the power signal on the base of the technology. The solution that I found, its to operate on the frequency of the 802.11n with one antenna pre-amplifier, so after the transmission of the main board antenna 802.11n, and let it to extend the range of the signal.
That Amp you posted an image of is made by a company called Edup and the build quality is very good.
I have the 4w 2.4ghz version which i use for amping up one of my RC transmitters, we need bidirectional amps because our transmitters are actually a link with telemetry and not a one way transmission to the craft.
Another well made 2.4ghz Bi directional amp with pre amp is the 2w Turbowing RY-2.4 i have a number of those and go several miles out with failsafes not being even close before my 5.8ghz video is crapping out on me
Forget the 1w many sellers state it as being, its got a 2.5w capable chipset but turbowing run it at 2w.
I know we need 5ghz for the Wii U but i thought id share my experience of other amps for anyone who might be looking to amp 2.4.
The big problem we have with amping up the Wii is its output power, do we know what it puts out?
These are all linear amplifiers which output power based on input, they have a min input of 3dbm (2mw) which will amp a little and a maximum of 20dbm (100mw) for the amps max output but that max input MUST NOT BE EXCEEDED otherwise you will overdrive and kill the amp very very quickly.
Regarding antennas, FPV antennas are tuned for 5.8ghz, this may be too high for the Wii U, the swr could be off the scale and make range worse or even kill the RF module but if its designed to work across the entire 700mhz range of the 5ghz band then they might be ok but would still be tuned for the top of the band when you would want something tuned for the center.
The other thing to note is to not buy anything FPV that is Circular polarized, no cloverleafs, helicals, pagoda ect, the Wii U uses linear polarization and using CP on the Wii U while the gamepad has Linear antennas will result in a huge loss, there are many linear FPV antennas though and i have a couple of double Bi Quads for 5.8ghz here with a 12-14db gain.
Even CP to CP will perform worse than linear due to the way CP transmits in a spread out corkscrew pattern vs on a single plane with linear it has a 3db loss compared to Linear just because of the polarization type, almost nothing outside FPV uses CP, pretty much only GPS, the Wii U mitigates phase loss due to gamepad orientation by using diversity.
I will crack on with installing external sockets fairly soon, i have loads of u.fl/ipex to RP-SMA pigtails here, they are also a much higher grade of coax than used in the Wii U, coax losses are huge at 5ghz and the coax matters!
These are my 2.4ghz amps: