Around 3x when docked and +50% undocked I think.
Splatoon 2 evidence that the console can run the game at 2x resolution docked and same resolution undocked at the same time that improve graphics significantly (Modeling, lighting, shadows, reflections...) and maintaining the rock-solid 60fps.
In terms of GFLOPS is around 2x docked and similar undocked in regard to Wii U, but the leap in terms of architecture (Old AMD DX10 and pre-GCN to actual nVidia Maxwell with DX11/12 feature level) is tremendous. Also, 3x RAM usable for games and a relatively modern ARM CPU compared to the obsolote Wii U CPU derived from GameCube.
In terms of GPU it can be around 1/3 of PS4 docked and 1/6 undocked but the more complex games may be even more limited by amount of RAM, bandwitch, CPU power... Especially the engines with high use of deferred rendering like Frostbite may be unportables by the very poor bandwith.