See this is the part that gets me. How is it that Sony possibly got the jump on MS in terms of rumored specs? I understand MS is going for the set top but...
i think it's reasonable to expect kinect to come packed in. the device's raison d'etre is to become emperor of the living room, injecting microsoft's metro-oriented ecosystem into your family's faces at every waking opportunity.
so if you've got kinect in there and mass market designs, that already leaves the spec budget looking a little slim; then comes the issue of memory.
to keep a veritable living room OS chugging away in the background, you don't need fast memory, you just need lots of it. PCs deal with this by splitting their memory in to slow general purpose RAM and fast video RAM, but it's an expensive solution not generally suited for consoles. microsoft can't afford to field 8GBs of GDDR5, so they've gone back to their design of slow system ram being complemented by on-die eSRAM/eDRAM.
the result of this is significantly reduced bandwidth compared with the potential of GDDR5, given a sufficiently wide bus. rumours state 100 GB/s vs. 192 GB/s for the PS4 (though i believe it'll be closer to 170).
then we get to the GPU, where microsoft are already painted in to a corner. price ceiling prohibits them from some monster bleeding edge chip and bandwidth limitations mean even if they did pack in a monster bleeding edge chip, it would be severely bottlenecked.