^- This. It isn't about the routers speed but the weak Wii U hardware. With a LAN adapter you'll reach around 30 Mbit/s but that's it.My guess is that the download speeds are being bottlenecked by hardware/the wiiu itself.
We constantinuously try to put load off the hardware so it has more resources to download (for example we climbed from around 27 to 30 Mbit/s lately) but I don't think there will be any major speedups anymore.
See it the other way around: NUSspli is downloading faster than the eShop since a long time. A lot of minor speed improvements followed after this major step.
//EDIT: And yes, Wii Us WiFi is 2.4 Ghz only.