Actually from what i can read. The wii Cpu has the same architecture as the wiiu and is stated to be backwards compatible. Only clockspeed is higher and it has 3 cores, also it has edram and faster/more cache. So i guess they just underclock it for wii mode.
About the gpu i'm not sure, it might have the ability to do the same work as the ngc/wii gpu.
--------------------- MERGED ---------------------------
The Wii U uses a custom
multi-chip module (MCM) developed by
AMD,
IBM and
Renesas in co-operation with
Nintendo IRD and
Nintendo Technology Development. The MCM combines an "
Espresso"
central processing unit (CPU) and a "Latte"
graphics chip (GPU), as well as a
SEEPROM memory chip.
[75][76] The Espresso CPU, designed by IBM, consists of a
PowerPC 750-based tri-core processor with 3 MB of shared
L2 cache memory and clocked at approximately 1.24 GHz.
[77][78][note 1] Despite belonging to the
PowerPC family, the Espresso also shares some architectural concepts with the
POWER7 architecture, such as the use of
eDRAM cache and being manufactured at a
45 nm node.
[79][80][note 2] The Latte graphics chip contains both a "GX2" GPGPU, which runs Wii U applications, and a "GX" GPU, which enables backward compatibility with
Wii games. The GX2, designed by AMD, is based on the
Radeon R600/
R700 architecture and is clocked at approximately 550 MHz
