You can't have just SPEs though, and a 1:4 Cell will have the PPU, memory, yadayada. SPURSEngine is supposedly 10-20 watts at 1.5GHz at 45nm. Just for the processor, where handhelds are wanting single digit wattages to last a few a hours.
The 15w was for a SPURS @ 65nm, and it has a lot more in the package than just 4 SPEs. I asked the question earlier in the thread, but nobody seemed to want to even give a hypothetical answer. If SPURS can give 48GFLOP @ 1.5Ghz, what would it give at 500Mhz or lower and 32nm or less, and what would the power consumption be? Obviously, it's not going to be SPURs, but it could be something similar. Host controller could be an ARM instead of a PPU.