Reminds me of the 130V lightbulbs sometimes sold as "consuming less power" (in the US, with 110V mains.)
Sure, they consume less power. But you don't get the rated wattage of light output either. In fact, you probably move the bulb into the range where you get fewer lumens per watt, for overall less efficiency.
You can divide most power-consuming devices into two classes: ones with smart power supplies that consume essentially constant power regardless of input voltage (most electronics), and ones that consume less power with lower voltage but also produce less output (light bulbs, heaters, motors, etc.) Most of the latter with still need to do fixed amounts of work, so your heater will need to run longer to keep your house warm when operating at a lower voltage.
Combined with the fact that a voltage-converting device will have its own inefficiencies, I vote: Snake Oil!