Since I suspected SoCtronics to be involved in the Wii U design, I dug a little deeper. Turns out at least one of my early conclusions was spot on: SoCtronics did, in fact, work with AMD Hyderabad on a GPU. And at least one more company was seemingly involved:
Incube Solutions. Turns out Incube Solutions designs high performance audio DSPs (Incube APS), as well as custom realtime operating systems, drivers and frameworks for high performance multimedia applications.
The part they worked on is a 40nm GPU with GDDR5 interface, ~625 million transistors, which would put it roughly in line with Redwood (5570). Raw performance wise, that part would actually be about twice as fast as Xenos.