John Carmack himself gave a good example on what might be an Xenos advantage:
"Embedded dram should be a driving force. It is possible to put several
megs of extremely high bandwidth dram on a chip or die with a video
controller, but won’t be possible (for a while) to cram a 64 meg
geforce in."
But again, the supposed higher Shader performance and the EDRAM advantage is all known factors and it´s been like this for a long time. That hasn´t stopped other developers from getting the same performance.
The real question is if id i able to get the Cell to do it´s part.
Carmack was talking about scratchpad in EDRAM, Xenos does not have it.
You can only resolve to EDRAM on Xenos, not from EDRAM.