I assume 1 6-pin and 1 8-pin connectors don't necessarily mean it needs to consume more power. The GTX 295 uses WAY more power, and probably more current electricity, but the HD6970 don't consume or need as much power, rather I assume is for design reasons or to prevent people from unlocking it? I'm sure 2 6-pins are enough to power an overclocked HD6970 with tweaked volts no problem.
You've only unloocked the shaders, the 6970 runs at higher memory/core speeds from stock, so draws more juice, although I suspect he extra 6+2 is a bit of corporate caution rather than engineering neccessity.
BTW, mine runs fine with unlocked shaders at 880/1300MHz just beware the full flash to 6970 specs, there have been reports of problems with the memory faiing under the higher timings/voltages.