If I load up all CPU cores on most of these systems, the wattage figures do go up. But that's not the normal 95+% use case. So the figures I've quoted above don't reflect full 200% CPU loads.

The MythTV box has a Core2 Duo 2.6GHz CPU, my notebook has a Core2 Duo 2.4GHz CPU, and the AMD Fusion board is an E450 design at 1.65GHz.

They all have SpeedStep or Cool'n'Quiet enabled, and so normally are clocked lower than the max GHz ratings.

The Core2 systems are now at least two generations out-of-date, and I'd expect newer designs to be more power efficient than those.