It is possible to run a lower voltage battery, most solid state electronics use some sort of voltage regulator to step-down voltage to 'logic' levels.

The idea of more torque is only possible if the current draw is more (since voltage is lower). This is counter-intuitve to extended battery life as it is the current draw that kills a battery (that is why watch batteries last continuously for years).

The 6V battery he is using is very likely a higher capacity (mah) than the 7.2V replaced, so battery life extention is possible. Under voltage sags from motor loads for instance can damage circuitry, but if you can use his tank as a test to verify if it works, then no skin off your back!