That's not true.  At 880mhz, I'm getting 379.8 mhash/s for each core (759.6 mhash/s for the card).  With finely tuned over/underclocking, I was able to get 3x6990 to do a stable 2.41 ghash/s in under 1300w for the entire system (efficiency of 1.85 mhash/J for the entire system, 2.00+ mhash/J for each card).  I'd guess the system w/o the GPUs is around 100-150w, which means each card is doing about 400w, maybe less.  I can overclock them even higher as well but I wanted to keep the system at safer amperage levels.  This was before software tweaks I made that pushed it 3.5-4.0% faster (3% of it thanks to bitless's Ma optimization).  I don't know how much power they use now but they're doing 2.50 ghash/s now.  I don't think power has changed too much since temps are about the same as before (all but a "defective" core are below 80C on air cooling).  I still have plenty of room to get it even faster but I'll be losing a lot of efficiency in the process and I'll be over the power usage constraint I placed upon myself for the whole 3 card system.

On the other end of the spectrum, I can tweak the system to be at least as efficient as 2.10 mhash/J (or over 2.35 for the card) while still maintaining a hash rate above 2.0 ghash/s.  I'm sure all the efficiency numbers are even higher now with the 3+% speedup.  The PSUs and GPUs matter quite a bit at the higher OC levels.  I've seen GPU cores that require a lot more voltage to achieve the same rate as another one.  And if you're unlucky (like me), you'll get a card with cores that are well over 10C higher than the other core on the same card, making it so you can't overclock as much while using up more power due to the extra heat.

I run Linux and use the phoenix miner with the poclbm kernel.  It's the fastest miner for me and beats even hashkill in raw speed while having less than 1% stales.  My numbers are now higher than hdminer now (@915, I get 817 mhash/s vs hdminer's 802. @960, I get 857 mhash/s vs 840).  I think my hardware tweaks put them over the top, as the software tweaks haven't quite reached the 6.4% hdminer claims so I'd be interested in finding out if hdminer would get even better numbers with my setup.  I'm just not willing to pay the hefty price to find out.