This is pretty interesting. I've been expecting binary compatibility since the start (much cheaper than including a Broadway, and much more reliable than emulation), and the write gather pipe and L1 locking are good finds, and seem to confirm this.
On the toolchain support, I wouldn't pay much attention to what other CPUs are supported, as the custom instruction set means they'll have to write a compiler and tools specifically for the Wii U's CPU anyway. In fact, I'd say the lack of Power7 and Cell support are simply down to a lack of clients requesting them (Cell is almost non-existent outside of PS3, and Power7s will be running IBM software most of the time).
There are three problems I've always seen with a design based on the 4xx series of processors; the lack of SMT, the lack of AltiVec units, and the low clock speeds the architecture is designed for. The lack of SMT you seem to have accounted for, and I'd accept the possibility that they could have squeezed 2.5Ghz or so out of the design, but the lack of AltiVec units is still something I believe rules a 4xx-based chip out. A good SIMD unit is simply too useful to game programmers to be left out of a modern gaming CPU and I definitely expect a heavily customized AltiVec unit, like a more modern VMX128, to be part of the design, something which
this IdeaMan post seems to indicate is the case.
In general I think people are searching too hard for existing chip designs to compare the Wii U's CPU to, and in reality we're probably going to get what is, functionally, a completely new chip. Nintendo went with a customized variant of an existing chip last time around because a suitable chip already existed, but with the Wii U that isn't the case. Designing a new CPU for a gaming console isn't exactly new. The PS3's Cell was a new design. The XBox 360 was originally supposed to have a new, out-of-order chip until what I assume were time constraints forced them to a more conservative PPE-based design (with the VMX128 seemingly carried over from the more customized chip).
What's more, since Apple switched to Intel, Nintendo have been IBM's biggest customer for CPUs, and the two companies have been working together for about a decade and a half now. Nintendo also have the money to spend on the R&D for a new CPU (and we know that Nintendo's R&D spending has been very high for the past few years), and there would have been enough time for a new CPU to be put together, given the apparent 2009 start of the Wii U hardware's design.
