I'm not sure where the " <0.5W/GH/s underclocked" claim came from.
It came from Simon himself:
Chip Specs
- 28nm custom ASIC
- Power Consumption: significantly less than 1.0 W/GH at nominal clock speed
- Designed to be underclocked for greater efficiency better than 0.5 W/GH*
- Designed to be overclocked for greater performance - better than 500GH/s**
* Real silicon power consumption may vary from simulation results by plus or minus 20%
** May require improved power supplies in the chassis