I felt compelled to register and share my experience.
I have a 6870x2 (basically a 6870 crossfire in one card, akin to the 6990's of the world, but with slower chips), and I'm having the same problems, as far as GPU usage. I'm using MSI afterburner to monitor it on screen, and it varies wildly but rarely ever breaks 80% usage, except on the awards screen after the round is over (when it shoots to over 90%). My specs:
Intel i7 860
8GB Mushkin DDR3
EVGA P55 MoBo
750Watt Corsair HX
ZR24W Monitor using DisplayPort @ 1920x1200x60
I'm currently using the 11.10 Catalyst final with 11.9 CAP 4's and that improved things quite a bit from the CAP3's and, well, everything else before it.
I've disabled HT, and that...... I'm not sure it improved much (I guess a bit, since I don't get the unexplainable lag spikes from the CPU, which weren't the bad, but oh well..... I'm turning HT on again tonight after upgrading yesterday and double checking that).
What did improve my system tremendously, despite some people who say otherwise (not to say they're lying, but they haven't experienced what I've experienced on my system) was adding 4GB of RAM. I did that yesterday and I don't get any heavy stuttering (near hard locks). RAM usage was reaching over 90% before, so that has been remedied.
I'm playing on everything ultra, except MSAA (which visually isn't making much difference right now, but my FPS, along with my GPU usage as reported by MSI Afterburner, varies wildly).
I'm thinking of trying this: http://hardforum.com/showthread.php?p=1037952451
and reporting back, but other than that, I've seen no other solution, and I'd really love to be able to run at my monitor's native resolution. I will keep an eye on this thread!