You're using a comparison that doesn't have a 7870 or 7850 in it, to prove something, and you're completely incorrect. The 7850/70 are SIGNIFICANTLY more efficient than the 680gtx. The 7950/70 are less efficient(more efficient in some games) but are an architecture aimed at something completely different(useless to gamers, but there is a reason for the higher power consumption the big kepler WILL have significantly worse efficiency than the 680gtx).
http://www.techpowerup.com/reviews/MSI/HD_7870_Twin_Frozr/24.html
If you notice, a reference 7870 use 103W to a 680gtx's 166W.
It's using over 60% more power for..... 35% more performance(it uses 1920x1080 extreme preset in Crysis 2 for its power testing), sorry it is not even close to more power efficient than a 7870.
it does offer less performance, but its less efficient. 7870's in xfire both perform significantly higher than a 680gtx, and won't use much more than 15-20% more power.
In terms of the 7850, the 680gtx is 60% faster, but uses 90% more power to achieve it, ballpark same efficiency as the 7870.
http://www.anandtech.com/show/5699/nvidia-geforce-gtx-680-review/8
this is much harder to see as Anandtech give complete system power, but in Metro 2033 the 680gtx is 32% faster than the 7870, yet uses 52W more power.
http://www.anandtech.com/show/5699/nvidia-geforce-gtx-680-review/19
Without hard numbers its hard to say if the 680gtx uses a little more, or the 7870 uses a little less in Metro vs Crysis 2, but a 52W difference is pushing 50% more power usage, for noticeably lower than that in terms of performance increase.
Meh. The power difference is insignificant for the performance of these cards though. Like I said, its nothing the same as a 560 ti vs a 480.
166 watts for a GTX 680 single card performance vs 103 watts for a 7850, that doesn't convince me that a pair of 7850s would be a wiser choice than one GTX 680, or even one of those VTX 7970s for £359.

What next, planting rhubarb and adopting snow leopards?
I like the idea of a super-efficient card, as I don't game all that often and not above 1080p