We need to assume that both MS and Sony have done their maths and optimised their architectures for end to end efficiency
If MS had shot bandwidth, they wouldn't need a decent GPU because it'd just be sitting there waiting for data, and that would be a waste of money to include it. So you have to assume that along the pipeline overall, DDR3 with 60-70GB/s is enough
The fun part is how the edram works, how much of the rendering pipeline can genuinely use that to save main memory bandwidth, and what special sauce there is. (Somewhat offset by promises of magic from edram with 360 that didn't entirely pan out)
All plenty of unclear details to fuel crazy speculation. Love it