You're getting way too excited about this, calm down. I think the GTX 780 has a bit more leeway at stock voltage, but if you over-volt the kepler chip is just as bad in terms of power consumption. The thing is, a lot of GTX 780 cards don't have OV capability and won't get super high overclocks, but will get moderate overclocks. I would put the SC ACX in this category. Once you over-volt the kepler, though, the same rule applies. Power consumption through the roof.
I think the main difference with the 290X is that the built in OC mechanism automatically ties over-voltage with overclocking. Therefore as far as I can tell there isn't a way to overclock at stock voltage (which is what the 780 SC ACX does) unless 3rd party utilities support the 290X.
So, essentially, the picture looks like this. I think the 780 has more OC headroom at stock voltages for moderate overclocks. This will allow typical 780 boards to reach 1100mhz, such as the SC ACX. That's actually the card i'm using as well, and I like it quite a bit. I think what these other guys are saying though is that Kepler uses a ton of power as well once over-volted - but only a few (2 or 3) GTX 780 cards support OV capability. Or you can use a modified BIOS to get up to 1.3V on any 780 card from what I understand - so basically when you over volt either the 290X or the GTX 780, power consumption will go through the roof. But most 780s don't have OV capability beyond +37mV unless you modify the BIOS. The point remains though, these guys are talking about over-voltage and you're talking about stock voltage.
Of COURSE stock voltage overclocks don't use a lot of power. No joke!
But once you modify voltage with either chip - 290X or GTX 780 - they will both have power consumption through the roof. I've seen multimeter readings of
780 classifieds using +200W from overvoltage with 1.35V. Same concept.