Ah, the old snake oil debate. Certainly "Voltage Optimisation" can reduce consumption with some uncontrolled (e.g. always on) fixed resistance appliances - e.g. conventional filament or old magnetic ballasted fluorescent lighting - but at the expense of lower output.  Generally the same effect could be produced by reducing the number of fittings slightly - but a VO unit may well be more convenient to install.  Tends not to work with heating as that's normally thermostatically controlled and the reduced voltage just means it takes longer to warm up - eventually the total energy consumed will be the same (or possibly even higher if the extra heat up time means greater  thermal losses). Anything with a modern electronic PSU (e.g. LED lighting) will almost certainly increase the current draw to maintain the same power demand, so no gain at all - in fact the increased current will likely mean greater losses in the wiring systems (and in extreme cases, possibly causing mild overloads).

    -  Andy.