I think , that power part of the design is not going to be bottleneck , here.
The SPI will be pain in the ass, You will have to use some kind of level shifting.
RPI is too much of a mess for my taste too
Well, Bitfury (corporation) and Punin (person) promised to release the specifications for their communication controller/level shifter/differential transceiver chip.
Bitfury Designs released under CC-BY-SA
https://bitcointalk.org/index.php?topic=1360038.0Maybe sidehack has some information in English now? I vaguely remember seeing some posts in Russian that are now deleted, and I'm only quarter-literate in Russian.